How do I make a texture variation for entities like with item_models?

for example, with commands you can give yourself an iron sword (or something of the like) with an alternate texure in a resource pack with the [item_model=xyz] part of the command, HOWEVER how do I do the equivalent with entities like for example a zombie?, I would like the geometry or shape of the entity to remain the same but just the texture to change with a specific /summon command, but also where would I store the texture in question in a resource pack?