In Minecraft Java, I am creating many resource packs by myself (no studio or external resource) so, I want to ADD a new file/entity/element/items/etc. to the game to not replace an old item because EVERY item/effect/entity has SOME use in the game. In conclusion: how I can ADD [not replace] a file in the game and give it custom physical and mental properties?
Get a texture for the Minecraft mob (or make one yourself).
In MCreator, Click on "New Mod"
Next, click on "Mob" and name your mod. Names mustn't repeat. Make it something you understand, the end-user won't see this name.
Then, fill out all of the custom characteristics of your mob.
Finally, export your mod into your preferred version of Minecraft.
I found these instructions from here.