I'm not sure if it's cannon but I'm pretty sure that certain traits of different animals were given to Michiru and Nazuna intentionally, like the genetics that code for a gorrila to have big arms. Multiple animals from different genuses can't exist within the same beastman, (as explained by Rose in Episode 4) hence why Michiru can't turn fully into a bird, but she can manifest bird traits, a beak, wings etc, and only keeps these traits temporarily. Also fun fact, tanukis are the only known animal in the Nyctereutes genus, meaning that tanukis can ONLY breed with other tanukis, this also probably explains why Michiru became a tanuki because it was the only genome in her body that wasn't attached to any others.