Looking back through some of my remaining notes from the late 70s (I've been up in the loft smile ) I find that our "A level Geology" study group looked at the development of the horse.

Between about 54 my and about 4.5 my, there is a developmental sequence from Hyracotherium (originally found and named in Britain; later discovered in US and called Eohippus) through Miohippus (absent in Britain), Parahippus and Pliohippus to Equus. The last of these gradually becomes indistinguishable from the modern horse.

Like dogs, but in a much less extreme form, horses have since been subjected to selective breeding by humans, but such would not be included in any record of fossil forms.

There never was nothing.