Need help making a custom mob

Hello!
I'm looking to create a custom wither skeleton that's using a texture from a specific resource pack. I want it to be visible without the need for OptiFine/Sodium for a small server I'm running for my friends.
So far I've figured out that I could do it by giving it custom items to wear that have models with the desired texture on. Unless there's a simpler way?
Either way, I genuinely have no idea how to go about it, I'm just starting out with datapacks.