At lvl 5, a Paladin gains the ability to summon their Paladin mount. However, at levels 6-8, they gain the ability to summon a different, unusual mount. At lvl 6, a Paladin gains the ability to summon a Unicorn as their special mount.
My question is: Is there any way to eventually upgrade this Unicorn to be a Celestial Charger? Of course not (necesarily) directly at level 6.
What you can do is take the feat Celestial Mount (Book of Exalted Deeds, 42) and add the celestial template to your unicorn. You can also get it by taking a planar substitution level at 6th level (Planar Handbook, 34) but this makes you lose the remove disease class feature.
The Celestial Charger (Monster Manual, 250) - a unicorn with the celestial template, 8 HD, 7 levels of cleric, and a challenge rating of 13 - would be by far too powerful as a special mount.
In addition to the Celestial Mount feat or the planar paladin substitution level mentioned in Peregrin Took’s fine answer, there are a couple of other possible options to get a celestial charger companion, though it is not a paladin special mount and does not benefit from that class feature (it could be in addition to a special mount, including a unicorn). On the other hand, these are (maybe) the actual celestial charger, i.e. the one that’s also a cleric 7th.
I notice that the celestial charger entry in Monster Manual includes
Level Adjustment [...] +8 (cohort)
The book never explains what “(cohort)” LAs mean, but the general assumption is that these creatures have this level adjustment when chosen as a cohort through the Leadership feat (and that they aren’t otherwise playable). The celestial charger has 8 unicorn HD and 7 cleric levels, so it’s got character level 15th, and an LA of +8 makes it an effective character level of 23.
Leadership says “Regardless of a character’s Leadership score, he can only recruit a cohort who is two or more levels lower than himself,” so in order to recruit an effective character level 23rd creature, the leader must be 25th level, i.e. an epic character. Still, this is an option.
Another option—maybe—is the beloved of Valarian prestige class from Book of Exalted Deeds. We already have a Q&A about how to become one as a paladin, but this question raises a new issue I hadn’t considered when I answered that question: what exactly does the call celestial charger feature do?
Call Celestial Charger (Su): At 6th level, a beloved of Valarian can call a celestial unicorn—also known as a celestial charger—to her side. See the Unicorn entry in the Monster Manual for the celestial charger’s statistics. [...]
(Book of Exalted Deeds pg. 54)
My initial reading of this feature is that it gets you a celestial unicorn, that is, a unicorn with the celestial creature template applied. That is, of course, what it says here in the outset, you can call a celestial unicorn. Its description of “celestial charger” is that this is just another name for a celestial unicorn. As I said in my other answer, the celestial template doesn’t really do very much for a unicorn—personally I wouldn’t even spend a feat on it, though I would probably be willing to trade remove disease for it.
But Book of Exalted Deeds explicitly says to use the statistics of the celestial charger in the Monster Manual’s Unicorn entry. And that, as Peregrin noted, says
The celestial charger described here is an 8 HD celestial unicorn with seven levels of cleric.
[...] Challenge Rating [...] 13
Beloved of Valarian’s requirements force a character to be 7th level before they can get in (the BAB +7 and Ride 10 ranks requirements each force that minimum), so a 6th-level beloved is, at minimum, a 13th-level character. Which exactly matches the CR of the celestial charger at the level you get it. It may well be that the authors of Book of Exalted Deeds intended for you to get this at this point. Which would be terrible, from an editing stand-point, since the wording on call celestial charger is miserably poor, but Book of Exalted Deeds has a reputation as one of the worst-written, worst-edited books for 3.5e, so this is kind of par for the course.
Now here is the question: is this reasonable? Peregrin opines that the celestial charger would be far too powerful as a special mount, and if we’re talking as just costing an adjustment to your effective paladin level (as with unicorn itself) or a feat (as with Celestial Mount), I’d completely agree with that. But beloved of Valarian costs way, way more than those things—it completely destroys your special mount progression and replaces it with the celestial charger, and it also fails to progress some other paladin stuff. More importantly, it requires three feats just to get in—which for many paladins will be literally all of their feats at this level—and two of those feats are absolute garbage. Plus there’s some really specific roleplay requirements. For all that, its class features are OK, but nothing stellar—I was happy to give them away in a series of feats, and that’s a pretty well-regarded answer.
And honestly, it’s a lot of jumping through hoops. A 7th-level cleric isn’t nearly as strong as a 13th-level cleric, even if it’s also a unicorn and it comes with a quasi-paladin attached.
So this might be reasonable. The celestial charger is probably more powerful than the beloved herself, but frankly that’s kind of the norm for paladins—especially for single-class paladins, the special mount is often the most powerful feature they have. It’s definitely the most optimization-friendly, and you can produce mounts far more powerful than the celestial charger.
The biggest problem is that the celestial charger doesn’t grow at all. At 13th level, it’s pretty solid, but at 20th level it’s looking quite poor.
This fine answer mentions the exalted feat Celestial Mount (Book of Exalted Deeds 42) and the level 6 paladin planar substitution level that grants the natural ability celestial mount (Planar Handbook 33), both of which add to the paladin's special mount the template celestial (Monster Manual 31), and either can gain a level 6 paladin a celestial unicorn special mount. However, such a creature isn't a celestial charger (249–50) as it lacks the charger's cleric class levels. Further, exalted feats come with baggage that's too heavy for some campaigns (see BE 39), but my reading of forums leads me to believe that many DMs ignore this, as I sometimes do.
This other fine answer mentions the prestige class beloved of Valarian that gains at level 6 the supernatural ability call celestial charger (BE 54) that may or may not allow the character to bring forth a real celestial charger with cleric levels and everything. This reader agrees with that answer's assessment that allowing the call celestial charger ability to bring forth an actual celestial charger is—in a typical campaign—not unbalancing, especially once the party's level is in the late teens or higher. Also, like exalted feats mentioned above, exalted prestige classes may come with heavy and possibly campaign-inappropriate baggage (see BE 49), but this, too, is sometimes ignored.
(The advantage—if one can call it that—to the call celestial charger ability lies in the paladin/beloved of Valerian being able to have present simultaneously her special mount and that celestial charger. (To be clear, a "companion" unicorn—like the unicorn animal companion of a ranger 14 with the feat Exalted Companion (BE 42)—is transformed by the poorly written call celestial charger ability, not a special mount.) However, like that answer notes, the prestige class doesn't improve the paladin's special mount. This means that the special mount'll likely be quickly slain by level-appropriate foes. So it goes.)
The Dungeon Master's Guide on Paladin Cohort Mounts says
At the DM’s option, she may allow a paladin or other character with a special mount class feature to combine the special mount with the cohort feat [i.e. typically the feat Leadership (106 and also Player's Handbook 97), but there are others; see here and below]. The special qualities such as the empathic link and shared spells make this quite potent and worth a minimum of a +2 level adjustment to the cohort mount ECL. (Dungeon Master's Guide 200)
The celestial charger has an effective character level (ECL) of 23 (equal to the charger's Hit Dice (HD) from being a unicorn plus HD from being a cleric plus, as a cohort, the charger's +8 LA—its level adjustment for being what the game thinks is a powerful creature). Following the DMG's recommendations above, taking a celestial charger cohort and also making it a paladin's special mount adds at least another +2 LA. That gives the celestial charger a minimum cohort ECL of 25.
Thus the game thinks that a celestial charger cohort special mount is typically appropriate for a level 27 or higher paladin that possesses the feat Epic Leadership (Power of Faerûn 155, 156) and a leadership score of at least 40. So you know, in all but the most ridiculously, over-the-top conservative campaigns, this is, frankly, 100% absurd and the game is just straight-up wrong.
The feat Leadership allows a creature to try to attract as a cohort a unicorn as if the unicorn were a level 8 character (DMG 199). The template template celestial (MM 31) adds +2 LA, making it seems as if a celestial unicorn cohort equals a level 10 cohort. Add to that the DMG's +2 LA for being a special mount, and it's effectively a level 12 cohort. Thus a level 14 paladin that possesses the feat Leadership and a leadership score of at least 17 could, with the DM's permission, gain as a cohort then turn into a special mount a celestial unicorn, and, from there, when the paladin gains a level, the creature gains enough XP (104–5), and the leadership chart allows (106), the celestial unicorn cohort special mount can start taking levels in cleric. (Note that the creature could instead gain levels in warblade or wizard or some class instead or instead just gain HD; the DM determines how a cohort advances.) If in a rush, a feat like Improved Cohort (Heroes of Battle 98) can reduce the minimum paladin level by 1 by allowing the leader to have a cohort that's but one level lower than the leader.
