Preliminary testing of replacing the Summon creature reveals that the visuals can be overwritten by replacing the VisualResourceID and the VisualSet of the RootTemplate via Norbyte's lstools (Divinity Editor is unfriendly to changing out visual stuff). PhysicsTemplate might need replacing too.
There was a big problem with this preliminary test though; the Cow that I summoned had no animations, was Summoned as a Guest NPC (not controllable), and didn't attack when in combat.