Hi there! I've studied horse genetics for awhile as a hobby (I used to pedigree/show Breyers). I think your best bet with this might be the 'chimera' genetic mutation. Most of the time it ends up looking more like calico cats with three or more colored-patches, but as you can see:
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loginMight be just what you're looking for! Chimera has a lot of different effects in terms of color too so that might be your best bet <3 I'd recommend doing some research on that perhaps?
Another possibility is an extreme frame or splash overo. They tend to have a lot of white on one side, and especially with frame, rarely does the white cross over the back/withers to the other side.
Hope this helps some!