Well I was Testing this file for bedrock edition in the definitions folder that has data for all the entities and block entities and I Believe that they are used for attaching models and textures to entities. I tried to look deeper into this to discover another way to add new entities or entity types. I use the squid for my research . I move the "squid.png" file into a new folder called squid and renamed it to "squid_0.png". Then I made more files like squid_1 to squid_6 and added them to the squid folder. I changed the squid definitions file called "squid.json" and changed the default to squid_1 and put it in the position of variant 0 and made the real default in the assumed position of variant 2. I made an behavior pack to spawn 2 different variants with different sizes and they both only spawn with the default texture. I really want to see if anyone else knew about it and could help me. I think this method only works with mobs that already have variants built in but I hope someone can prove me wrong.