Poofah's done a really good job with pointing out some "overpowered" pets on their own. But in my opinion, nothing's really overpowered, until you start getting into pet combos.
For example, the Crawdad is indeed near to unkillable, but combine him with any pet that does Sunny Weather, and his Wish heal, that was already healing him for like 900, now heals him for like 1800. And the awesome 1800-1900 HP he had is now closer to 2700. Good luck dealing 2700 damage to him in 6 rounds with his damage reduction buff up.
Another example would be using a pet with Plagued Blood, like the Restless Shadeling, to keep that Plagued Blood buff up on the target and using another pet that has multi-attack abilities afterwards, making him heal on every hit. A good example for that would be the Clockwork Gnome. He can have up to two towers up, which each shoot 3 times per round, if the enemy has Plagued Blood, your Gnome will get a heal on each tower hit plus his own attack (so up to 7 heals in a single round, when timing is right). Keeping in mind the Gnome also has Repair as an ability, it makes him pretty unkillable too if you can keep Plagued Blood up.
There are several more examples like this I'm sure, but these are two pretty good examples of powerful pet combos.