Even if temporarily?Jack Simth said:It suddenly understands common, unless otherwise specified, and can make somewhat intelligent decisions, advance in class levels, et cetera.
Sejs said:I would definitely say it does not suddenly sprout the ability to understand common.
If it's thru magical means (awaken, say) then that already has a system in place - the animal changes type to a magical beast.
Hairfoot said:"Oh, my god! I'm a cat! A cat! Why didn't anyone tell me?
Do I have a soul? Is there a god?
*gasp* All those mice! Those poor, poor mice I tortured to death when I wasn't even hungry.
I feel so alone. Who can I talk to? Quick! Give me a stiff drink and a number for the sentient cat support group!
Gaaah! No thumbs!"

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.