Hiya!
Your title confused me; I read it, saw the poll, and voted "Yes". Then I was going to come here and explain why...but I read your OP to go with the poll and then realized that your Title should have been "Polymorph - Must have seen/encountered, or not?" or something like that.
I have no problem with any PC polymorphing into a t-rex, giant ape, or any other creature...as long as the PC has actually encountered it before. I mean, it's like me saying "I'll polymorph myself into an Ili Pika!"...have I ever seen one? Nope. Do I know what they sound like? Nope. Do I know what they smell like? Nope. Do I know what colour hair they have? Not really. Do I know the pattern, if any, that they have on their fur on their backs? Nope.
So..."Yes, if your PC has encountered one. Yes, if your PC has heard of one...but that requires a Nature check. No, if you are only naming t-rex because you, the Player, saw it's stats in the MM". And by "...has encountered one", that only counts in my game. Saying "Yeah, in a game that Steve ran...no, you don't know Steve...my PC was fighting dinosaurs all the time!"...doesn't cut it.
That said, we've never had a PC with Polymorph as a spell, so...yeah. I also allow Druids to Wild Shape into any animal that they've seen...and yes, that means a 2nd level druid can wild shape into a hawk and fly, or into a squid and swim, or into a rat, or a horse, etc. I pretty much ignore the 'movement' and 'size' limits of the ability.
^_^
Paul L. Ming