Polymorph in this instance does not refer to the spell, but as a verb describing what happened to the wizard
You have quoted the relevant text from the book, and the only bit of lore that really explores this transformation:
Traxigor was polymorphed into an otter years ago, and decided he preferred the new form to his original one (that of a wizened old man). His otter form was made permanent by a wish spell. He uses the archmage stat block, but is a Tiny beast with a Strength of 3.
I've retained the original formatting for this, as it is important. Notably, the spell, wish, is italicised while the word polymorphed is not. Wizards has taken the convention, in their printed works, to italicise words when they are referring to specific spells. In this case, we can take the lack of italisiation on the word polymorphed to mean they are using it as a verb (specifically the past tense of the verb) as opposed to a reference to a spell with polymorph in the name.
One other interesting feature of the wording is that it doesn't say he didn't transform back into his wizard form intermittently to perform the wish, just that he was polymorphed into an otter, and then at some point after that initial polymorphing he used wish to make the otter form from that polymorphing his permanent form.
Do WoTC use this "verb" form in any other contexts in their work? (to support this interpretation)
In any creature who is a shapechanger their Shapechanger ability has text of the form:
Shapechanger. The imp can use its action to polymorph into the beast form of a rat, a raven, or a spider, or into its devil form. Its statistics are the same in each form, except for the speed changes noted. Any equipment it is wearing or carrying is not transformed. It reverts to its devil form if it dies.
(I've quoted from the Imp statblock specifically, but the form of the ability is the same)
Similarly, in the text describing the Lythari elves, from the Sword Coast Adventurers Guide they are described as:
Lythari. The Ly’Tel’Quessir have the ability to polymorph into wolves. Unlike werewolves, lythari don’t have a hybrid form and aren’t afflicted by a curse. They dwell together in secretive packs, primarily in wolf form, living free in the deep wilds of the world.
Lastly in the DoTMM adventure (enclosed in spoilers), there is this "feature":
Throughout Arcturiadoom lie magic traps designed to polymorph humanoids. (Creatures that aren’t humanoids are unaffected.) These traps are marked P on map 14. The hobgoblins are aware of these traps and do their best to avoid them. A polymorph trap is marked by a nearly invisible glyph inscribed on a 10-foot-square section of floor. A character searching the area for traps can detect the glyph with a successful DC 20 Wisdom (Perception) check. The first humanoid to pass over the 10-foot-square area triggers the glyph and must succeed on a DC 20 Wisdom saving throw or be transformed into a monster with average hit points. All items worn or carried by the creature are absorbed into its new form. The new form is hostile toward all other creatures and must attack any other creatures it can see. Roll a d10 to determine the creature’s new form:
1. Carrion crawler
3. Fire elemental
4. Gelatinous cube
6. Hook horror
When the polymorphed creature drops to 0 hit points, it reverts to its original form and is stable at 0 hit points. Once a glyph is triggered, it disappears, rendering its space safe to pass through until Arcturia sees fit to replace it. A successful dispel magic spell (DC 17) cast on a glyph removes it.
This "feature" describes how someone would be magically polymorphed without using one of the two polymorph spells. Indeed it has some elements that are explicitly contrary to the polymorph spells themselves:
> When the polymorphed creature drops to 0 hit points, it reverts to its original form and is stable at 0 hit points.
This is explicitly contrary to how polymorph itself works, in which the creature reverts back to it's original form, with any damage transferring over to the full hit points of it's original form
> The new form is hostile toward all other creatures and must attack any other creatures it can see.
Under the polymorph named spells, the hositility or otherwise of the creature is not changed by the spell.
It very explicitly does not carry the "The creature is limited in the actions it can perform by the nature of its new form, and it can't speak, cast spells, or take any other action that requires hands or speech" restriction, so the newly polymorphed form from this "feature" will be capable of those things!
Ok, do we have any potential candidates for how this polymorphing might have happened?
Yes a few:
- Shapechange (cast by another spellcaster) followed by Traxigor casting wish upon himself to make the change permanent
- Shapechange (cast by Traxigor) followed by another spellcaster casting wish upon Traxigor to make the change permanent
- Using wish directly to change himself into a permanent otter form with the ability to cast spells
- He (or another spellcaster) used a spell like polymorph (but not one of the Polymorph spells) without it's limitations on casting spells that isn't available to the players. Remember the PHB says the spells printed are not the only spells possible in D&D, just
[...] the most common spells in the worlds of Dungeons & Dragons.
- His form was changed by a god or some other more powerful being without the spellcasting limitation, and then he wished to retain it.
Do we know which one of these was used, or if some other method was used?
No. The only place Traxigor appears is in Baldur's Gate: Descent into Avernus, and no further lore sources are given for his specific transformation.
The use of the word polymorphed in the description does not, and should not, be taken to refer to the two spells with Polymorph in the name, particularly given the clear inconsistencies of the character's abilities when compared with what would be possible for those two forms.
There are a variety of ways he could potentially have had his shape changed that would be consistent with his form, but we do not know which one it was, so absent some additional publication from WoTC, or a Forgotten Realms novel featuring Traxigor, we are not able to give a definitive answer. The only thing we do know is that he cannot possibly have used Polymorph or True Polymorph to achieve this state (because he can cast spells).