Pouches use the #wich:pouchable tag [all non-aquatic vanilla (and some modded (see below)) mobs smaller than a block by default]. Entities in the #wich:never_pouchable tag [empty by default] can never be pouched regardless of the game rules set.
The wich:allow_pouching_baby_mobs (allowPouchingBabyMobs before 1.21.11) gamerule [true by default] allows baby mobs (including small slimes) to be pouched.
The wich:allow_pouching_all_mobs (allowPouchingAllMobs before 1.21.11) gamerule [false by default] allows all mobs to be pouched.
Players cannot be picked up with pouches and pouching a mob will dismount it / dismount any of its riders.
Try out the Entity Item Tooltips mod for detailed tooltips on the mobs in every pouch.
The Pouchable Villagers example data pack allows Villagers to be pouched explicitly.
Pouches should work on all mod mobs but will need them to be added to the #wich:pouchable tag or allow pouching all mobs first. The pouchable tag is already configured to support mobs from:
External resources
Project members
OfficePigeon
Member