## A modified stat block is still a stat block

In summary, a modified stat blocks counts as a stat block, too; *polymorph* as written does not stop you from copying an individual creature's stat block, so you could copy a modified beast, but it would be up to the DM if that process also copies things like equipment of the target. 

### The game refers to statistics and stat blocks (or character sheets) interchangeably

*Polymorph* itself never uses the term stat block, it says:

> The target's game **statistics**, including mental ability scores, are replaced by the statistics of the chosen beast. 

However, there is a Sage Advice Compendium entry that explains that the *statistics* refer to a stat block:

> **Can a creature under the effects of *polymorph* have
other spell effects on them, or are those game statistics
also replaced by the those of the beast form?** *Polymorph*
replaces only the target’s character sheet or stat block with
the stat block of the chosen form. Other effects, such as
other spells, still exist

The DMG (p. 92) further states:

> When you give an NPC game statistics, you have three main options: giving the NPC only the few statistics it needs, give the NPC a monster stat block, or give the NPC a class and levels

And the Monster Manual says on page 6:

> A monster’s statistics, sometimes referred to as its **stat block**, provide the essential information that you need to run the monster.

So, having statistics and having a stat block (or character sheet) are interchangable. (It's another question what happens if you could turn into an individual NPC that has incomplete statistics, but I am not aware of an ability that allows for this. Presumably the DM would need to make something up in a hurry, or use Commoner stats or another standard stat block). 

From the game's perspective, a creature has statistics, and those are recorded or formatted in a stat block (or character sheet). If you modify the stat block, the creature still has statistics, just different ones. You again could print them as a stat block. 

There are also creature statistics that are not for individuals and that are defined in exactly this manner. For example, *Tomb of Annhilation* provides an actual stat block for an Albino Dwarf, but also defines Albino Dwarf Spirit Warriors like this:

> An albino dwarf spirit warrior has the statistics of an **albino dwarf warrior**, except it has a challenge rating of 1 (200 XP) and gains the following additional feature:
>> ***Innate Spellcasting***. The dwarf's innate spell casting ability is
Wisdom. It can inn ately cast the fo llowing spells, requi ring
no material components:
>> 1/day each: *hunter's mark , jump, pass without trace, speak with
animals , speak with plants* 

While there is no actual stat block printed for these Albino Dwarf Spirit Warriors, this description is equivalent to having a printed stat block for them, that includes the Innate Spellcasting feature, it just saves space for more adventure text, instead of replicating nearly all of the stat block. 

So, in principle if a creature has statistics, these can be represented as either a stat block as a reference to a printed stat block with noted modifications. For the purpose of a spell like polymorph, these are statistics.  

### What do the spells say?

However, if you can transform into something also depends on the text of the spell. There are many individual creatures that do have actual, printed stat block, for example the archlich Acererak in *Tomb of Annihilation* has a stat block. That does not mean you can turn into Acererak if you *true polymorph* yourself, because that spell says

> If you turn a creature into another kind of creature, the new form can be **any kind** you choose 

so you can only transform into a *kind of* creature with that spell, not into an individual creature. You could transform into an Albino Dwarf Spirit Warrior, but not into Traxigor,  a CR12 level spellcaster that also is a tiny beast (from *Baldur's Gate, Descent into Avernus*, kudos to @ThomasMarkov for finding this).

*Polymporph* actually does not limit you to a kind of creature, it only limits you to a creature of beast type:

>  The new form can be **any beast** whose challenge rating is equal to or less than the target's 

So technically, as long as it is a beast, and it has statistics, it could be a target. You could argue that if *Polymorph Any Object* cannot do that, than the much weaker spell *Polymorph* should likewise not be able to do it, and I can see DMs ruling like this, even if that is not what the text says. 

Note that playing it like that and being high enough level would allow you to transform into Traxigor.  A DM who dislikes such an outcome could limit the spell to just general kinds of beasts. 

### Its not clear if equipment would be created

https://rpg.stackexchange.com/questions/57124/does-the-true-polymorph-spell-form-a-creature-with-equipment/154993#154993 discusses if you can or cannot create equipment, with answers highly upvoted on both sides of the fence. Normal polymporph has this problem less often, because the normal beast stat blocks typically have no equipment, but could you polymorph, for example, into an **[armored sabre-tooth tiger][1]**, if you had seen such a beast, i.e. a sabre-tooth tiger in armor (like the one in *Curse of Strahd*)? The same goes for creatures that have been modified, for example by losing a limb. If you polymorph into that creature, will you have all the limbs? I think this is up to the DM. Equipment is listed as part of the creatures statistics, but stated designer intend is that it would not be copied. 

For my tastes, limiting transformation only to a kind of creature avoids these issues, and is what I would do, but another DM might approach it differently. 


  [1]: https://rpg.stackexchange.com/q/208485/75095