Possibly started out as a cave dwelling lizard-like species before emerging to take over the surface. Gravity no more intense than ours, possibly less - an adult Rezark is 21 feet tall. Their large glowing eyes grant them excellent nocturnal vision, and they can operate during the day or night which contributed a lot to their success.
They are very social animals and live in huge family structures, somewhat like ants. They are prone to mutations which result in different colors and abilities. It seems as if Rezarks are uniquely prone to beneficial mutations, but in actuality, if an individual is found to have a non-beneficial mutation, they are cannibalized!
Damn, so they're pretty brutal, eh? Well it helped them get this far so I guess it must work.
Eopod and Maldon are also game enemies, unlike Rezark they come from Earth - the Earth of an alternate dimension. They are both predators, hailing from an equivalent of the Jurassic period where gigantic animals are common.
That aside, I'm liking these ideas so far. Strange stuff, but certainly interesting. I like the idea of alternate Earth monsters. It leaves a lot of room to get creative.