Eohippus 18 pairs of ribsAs the horses go from smallest to largest, the pairs go from 18 to 15 to 19 to 18. Truly the "missing links" here are a case of "missing ribs." Why drop 3 pairs of ribs from Eohippus to Orohippus and then add 4 pairs from Orohippus to Pliohippus? This is more evidence that is conveniently omitted in textbooks.
Orohippus 15 pairs of ribs
Pliohippus 19 pairs of ribs
Equus Scotti 18 pairs of ribs
Source: Have You Been Brainwashed? by Life Messengers.