So Xanathar's Guide has given us the Find Greater Steed spell. This is in some way better than the Find Steed spell.
This can be inferred from the names of the spells and the sample mounts it gives us; Find Steed says:
...the steed takes on a form that you choose: a warhorse, a pony, a camel, an elk, or a mastiff. (Your GM might allow other animals to be summoned as steeds.)
And Find Greater Steed says:
...the spirit takes on a form you choose: a griffon, a pegasus, a peryton, a dire wolf, a rhinoceros, or a saber-toothed tiger.
My issue is with the fact that it is not clear what the difference between these spells actually is. It does not explicitly state that one of these spells has a type restriction (such as Find Steed only allowing beasts, whereas Find Greater Steed allowing monstrosities, for example; leaving aside that you then override that type with fey, fiend or celestial), having a restriction on flying or other speeds, or having a max CR in either case.
We can derive a max CR from the sample lists, see Dale M's answer to this question, but it isn't explicitly stated. For things like druid's Wild Shape this is explicitly laid out, but it is not made as clear for these spells.
However, the Find Steed spell description does state "(Your GM might allow other animals to be summoned as steeds.)" Also that Find Greater Steed lacks this kind of statement, implying that for Find Greater Steed, you must use on one those suggested, whereas Find Steed has more options.
So is the only difference an implied CR increase based on the sample creatures, even though Find Steed wasn't explicitly CR capped to begin with?
Or, is there nothing stopping the DM from allowing anything they like* for Find Steed, thereby making Find Greater Steed completely redundant?
This is also an issue for characters that already have more interesting mounts for Find Steed prior to the Find Greater Steed spell being "released", so such characters who were awarded, say, a Pegasus by their DM as a Find Steed steed would never have to consider casting the Find Greater Steed spell.
*Obviously the DM can allow whatever they like and override the rules completely, but even so the Find Steed spell specifically calls attention to the DM's power in this case.