[humblegunner]

I’m thinking it was a mini-horse, a proto-horse. Not what we think of as a modern horse.

[stormer]

The horse in question is Protorohippus venticolus, first identified as a Hyracotherium by Edwin Cope (a very important and especially interesting American paleontologist in the late 19th and early 20th centuries - "The Guilded Dinosaur" by Mark Jaffe is a very interesting story about the academic war between him and arch-competitor Othniel Marsh). May people would know this animal as Eohippus, the Dawn Horse, a dog-sized proto-horse with 4 toes on the forefeet and 3 on the hind.