This diagram (from the horse FAQ) just shows genus names. In 1940, this diagram would have showed only 15 genera, representing only 200 species. But we just keep finding horse fossils. There are entire books about them.
The numbers down the left hand side are the dates. The present is at the top, and 55 million years ago is at the bottom.
2My Old & New World Equus
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4My Hippidion Equus Stylohipparion
| | Neohipparion Hipparion Cormohipparion
| | Astrohippus | | |
| | Pliohippus ---------------------------
12My Dinohippus Calippus \ | /
| | Pseudhipparion \ | /
| | | |
------------------------------------------- Sinohippus
15My \ | / |
\ | / Megahippus |
17My Merychippus | |
| Anchitherium Hypohippus
| | |
23My Parahippus Anchitherium Archeohippus
| | |
(Kalobatippus?)-----------------------------------------
25My \ | /
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|
35My |
Miohippus Mesohippus
| |
40My Mesohippus
|
|
|
45My Paleotherium |
| Epihippus
| |
Propalaeotherium | Haplohippus
| | |
50My Pachynolophus | Orohippus
| | |
| | |
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55My Hyracotherium