This also means that : Rex may disappear as well when he dies.
Did the eskimos in level one also do that? They have a pelt that graduates from polar bear fur to tiger fur.
Fridge Logic: Why does Rex's enhanced abilities and glowing eyes only activate now?
Probably because his Heroic Willpower overrode the remote, and the controller likely accessed the powers Rex didn't know he had.
That plot element was steadilly growing through the story, gaining Rex new powers the more exposed to the technology he is, including being able to control the animals without the base functioning properly. He offhandedly mentions to Lucy some have been present all the way back in his war corresponder days.
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 make all flying units useless. You can put four of them right next to each other cheaply, and they will absolutely slaughter all the flying animals near by with electricity.
Whale mixes are expensive to produce, but they have a ranged attack (if you keep the whale head) and a LOT of hit points, meaning they are almost always worth it. One of the very best combos (perhaps even the very best combo) in the game is a whale with lobster pincers and legs. On top of the things already mentioned, you get devastating physical attack power, and heath regeneration.
Scrappy Mechanic: The game has serious balancing issues. Most of the animals are outclassed or are downright useless novelties because of the stats not being assigned in a balanced way, meaning your range of valuable choices is limited. Anti-air towers are far too strong, rendering a entire class of animals (flying) almost useless for anything other than scouting.