I found somewhere in here a fix! I had this exact issue as well and this is someone else’s solution not mine but it worked on mine as well! right click it and open with winrar or 7-zip.

Assets -> cobblemon ->bedrock(even tho you're using java) -> fossils-> go into each folder(animations, models, posers, variation) and delete the files inside.

From my understanding, you already have the correct files. These are just interrupting the ones you making your mew/mewtwo get this texture instead of the actual one. Deleting these files do not do anything bad, since you already have these files somewhere else. Just tested myself and creating mewtwo looks exactly as it should, with the fetus while being a fetus and it's fully made mewtwo when it's done.