Although some closely related species can interbreed (lions/tigers and horses/donkeys), their species look much too different.

That being said, Hojo could have quite easily created a chimeric cell by combining their cells together.

Also scientists have also have been able to breed a mouse that they can insert a human immune system into.

Using DNA manipulation it's quite easy to insert foreign genes into organisms of a different species. Examples include bacteria that make human insulin and tobacco plants that express a firefly gene and can glow in the dark.

These last two techniques involve modifying existing organisms. As for the first one, a single hybrid cell is not the same as a viable organism.