To riff on that, extend poem to sonnets and stanzas, make it about monsters, magic, and places that are tied to the lyrical.
Those are poems.
I Think it more interesting to attempt to describe magic monsters and places with poetry, but maybe that is too broad/boring.
I just like the idea of constraints - and poetry has rules and constraints.
A book of poems, each one describing a creature - and stats - that would be a great way to make descriptions interesting, and evocative, and maybe more open to interpretation than a usual creature book.
Maybe your idea could work for a setting - bards and such. When I think of riddle and songs in conjunction with d&d, I always think of the Hobbit - and in Middle Earth riddles and songs seem to be part of it on a deep level.