And why would it ever occur to them that the prehistoric artist would NOT be painting what he saw/
I’ve always liked the way the ancient artists “signed” their painting with a stencil of their hand. Reaching across 35,000 years, that ancient artist says “I did this”!
Other parts of the paintings are exaggerated or abstracted. (The horse forms in particular.) It doesn’t seem unreasonable to believe other aspects might have been abstracted from or emphasized beyond literal reality.
With no evidence of spotted horses existing at that one it wasn’t a totally unreasonable speculation.