I was playing a moon druid and cast the polymorph spell on myself, changing me into a T-Rex, which caused my Int to become 2 and my Wis 12. I realized my targets had immunity to non-magical damage which is all a T-Rex can do, so I wanted to stop concentrating. In-game, we had a character with the capability to speak to me in T-Rex form and successfully explained to me that as I am now I was useless and to stop the spell.
I made the check to determine that my current form could not affect the creatures, but when I stated that I would stop concentrating, the DM said that, as a T-Rex, my Int was too low for me to realize that I had a spell I was concentrating on that was keeping me in this form. So, I was stuck as T-Rex until the HP was depleted or the spell ended.
Was my DM correct or can a polymorphed creature still choose to end concentration?
Best Answer
There is no intelligence limit on the rules for concentration.
Nothing in the rules dictate that a creature has to have a sufficiently high intelligence to maintain or drop concentration. In fact, there are examples of extremely low intelligence creatures fully utilizing concentration spells. See the Neothelid in Volo's Guide to Monsters (p. 181):
If this creature, who is not much smarter than a t-rex, is able to use a concentration spell like levitate, there is hardly a reason a t-rex wouldn't be able to use one to full effect as well.
Your DM may be using a houserule.
It is possible that your DM is implementing a houserule where intelligence has more of an impact than in the base game. If this is the case, he is the only one who can answer questions like why a t-rex could/would concentrate on a spell at all.