I think it could be something like Cosmina was. One day it was a person, but Dorothea change it to monster. Dorothea likes doing experiments on people.

I just saw it on internet, it's in chapter 60. She used something like alchemy (similar to Fullmetal alchemist style) to summon this monster.

If it wasn't explicitly identified as a Danger Beast, we shouldn't label it as such.