There are two pools in making a zombeast:
The first pool consists of cards with complex effects, such as Cave Hydra, Webspinner, Lifedrinker, Tundra Rhino.
The second pool consists of cards with simple keywords or no text, such as Vilebrood Skitterer, Stonetusk Boar, Bloodworm, Snowflipper Penguin.
You cannot make a zombeast with two beasts from the same pool. That's the fundamental rule of zombeast making.