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)...
This should be most of them, though there are likely more monster abilities to find, as the DND Beyond search results are limited to 35 entries. If you find more, leave a comment or edit them in where appropriate.
Spells and spell effects
Spells and spell effects that name wish as an end condition:
Ending the effect of feeblemind
Reversing the hit point ...