I'm just getting access to the Polymorph spell this level, and I'm curious about beasts with innate capabilities. Swarm of Cranium Rats (CR5) are probably the strongest example:
Innate Spellcasting (Psionics). The swarm’s innate spellcasting ability is Intelligence (spell save DC 13). As long as it has more than half of its hit points, it can innately cast the following spells, requiring no components:
At will: command, comprehend languages, detect thoughts
1/day each: confusion, dominate monster
Do you have access to a monster's innate capabilities while you are polymorphed into them? I'm interested in learning the answer for all monster traits in general, but I'm particularly focused on spellcasting.