I was running a game yesterday and the caster decided to Polymorph one of the group members into a T-Rex. Is that possible. Character was level 8 and t-Rex is cr 8.

Did I miss something in the rules?

thanks for any help on this issue

Quote from Artorius >>

I was running a game yesterday and the caster decided to Polymorph one of the group members into a T-Rex. Is that possible. Character was level 8 and t-Rex is cr 8.

Did I miss something in the rules?

thanks for any help on this issue

 That is perfectly fine assuming a T-Rex exists in your world. Id also impose the limitation that the spell caster has to know of the creature to do so, but that isn't strictly in the rules. 

With any save, you can chose to fail if you want. There are some rare cases you may want to fail against harmful spells. 

Thank You for the information. Very helpful.