Arsthbt

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.

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 depleted or the spell ended.

Was my DM correct or can a polymorphed creature still choose to end concentration?

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):

INT

3 (-4)

[...]

Innate Spellcasting (Psionics).

[...]

At will: levitate

If the 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.

RAW implies you can end concentration, but a shaky argument can be made against

According to PHB, concentration is very easy to end.

You can end concentration at any time (no action required).

Additionally, the Polymorph spell states:

[The target] retains its alignment and personality. [...] The creature is limited in the actions it can perform by the nature of its new from, and it can't speak, cast spells, or take any action that requires hands or speech.

Nothing here says you can't end concentration, which seems to be a simple mental activity.

The one argument I can see is that the beast couldn't have started concentration (part of casting the spell), but that would imply the beast can't maintain concentration either, which would never allow you to successfully Polymorph yourself. If the DM is prohibiting ending concentration, he/she should prohibit Polymorphing oneself.