FormerlyHemlock
Hero
Depending on the Int of the creature you polymorph into, I would be concerned that it may attack it's party as well as the enemy. I may argue that the player is giving up some of its control to gain power and damage. Typically I always want to let the players control their character, but this may change what his character is for a time. I'm not sue how low Int needs to be before you no longer recognize your friends as such. One can argue that dogs won't attack their master or something, but when you Intelligence instantly becomes so low things may change.
T-Rex's Int 2 is more than sufficient. Even an Int 1 Feebleminded character can recognize friends and allies.
http://www.5esrd.com/spellcasting/all-spells/f/feeblemind said:On a failed save, the creature’s Intelligence and Charisma scores become 1. The creature can’t cast spells, activate magic items, understand language, or communicate in any intelligible way. The creature can, however, identify its friends, follow them, and even protect them.
If you take away control of a Polymorphed T-Rex from the player, you ought to be prepared to do the same for Feeblemind as well.
Also, if you take away control of Polymorphed characters, Transmuter wizards turn into garbage, since Polymorphing themselves into dogs/cats/birds/etc. is their 10th level shtick.