I don't think any pets are necessarily OP. I think there are some good pets and some not so good pets. The most complained about seems to be the Monk but people forget that in order to good damage he has to sacrifice a turn in which he does no damage. He is very easy to counter with a dodge or any avoidance move and umpteen million easy to find pets have them. Add in the fact that he has to sacrifice survivability to do top damage and I don't think he's OP at all. He's a heavy hitter, for sure, but it doesn't matter how hard he hits if it never lands.
Although I can't remember the name of it since I haven't been keeping up with my archaeology skill, the little flying pterodactyl that you can get through the skill absolutely tears up in the lower levels (1-10). Definatly worth taking the time to get it and use it against unsuspecting battlers.
I'm not sure how it compares to higher level battles, but I hope it evens out over time.
Pterrodaxx Hatchling! And she does great at max level. Decent damage, avoidance, and a nice heal. She's not a heavy hitter but she holds her own. And she's great against healer pets like crabs and frogs.