Where the minecraft entitys models are or how i download it and how i can put it in a mcreator mob?

Don't worry about the bad English. As long as you manage to get your point it, you'll be fine.

The Minecraft EULA prohibits players from sharing mob models online, so you'll most likely not find any there.

Your best bet is to download Blockbench and create a "new skin." Select the skeleton model and press create. Then you can export it and paste a texture.

This method isn't always the best. Your new model most likely won't have an animation, but if you're still testing, I don't think that matters