Lobsters are surprisingly useful. They've got nearly unrivaled armor, the ability to swim, regeneration, and exceptionally powerful claws. Mixing a lobster with a large creature (hopefully with a powerful head-based attack like a crocodile) leads to a creature that is not only physically very powerful, but also just happens to have an array of surprisingly useful special abilities.
The archerfish has a range attack that does a lot of splash damage. Just combine something big with one and watch them destroy herds of creatures. The attack also can be upgraded to do piercing damage, which makes it even more of a Game Breaker. The only downside is it's one of the few attacks in the game that can damage ally units.
Antiair towers pretty much make all flying units useless.