how to add a mob to Minecraft [1.16]

I have tried to search around google, Bing, and Minecraft forums. I wand to ADD a NEW mob to the game.

———— requirements for the answer ——-

1. it has to be compatible for 1.16.4
2. it has to be how to add a new mob not change an existing
3. no spam
4. it cannot be a mod to add this to Minecraft
5. I want it to be in my resource pack
6. If I have to create a new pack, that is fine