What next, tardigrade and amoebas?
Amoebas are out because they're not animals. There are ameboid cells *in* animals, but amoeboids as organisms cannot be classified as animals.
However, tardigrades would be interesting.
Currently, there have been no animals in Killing Bytes outside of the phylum Cordata. Mammals, birds, fish, and lizards are all Cordates. As noted in the series itself, the further away an animal is from a human, the rarer/more difficult it is to hybridize a human with that species. This is why birds are rare. Presumably, fish are also rare (we've only seen one shark). But all of the animals seen so far have had some type of spinal cord, in common.
We've seen no arthropods yet, for instance, such as an insect or a crab. But if we ever did see such a hybrid in Killing Bites, then yes, it would open up the possibility for yet other phyla to emerge, including tardigrades.
And if a tardigrade hybrid really was made, well, it'd basically be invincible. It wouldn't have much offensive capability, but killing one would be nigh-impossible, so it could wear out any enemy eventually.