"Transitional forms"
The evolutionary history of the horse is written all the way up from something the size of a cat. There were dinosaurs with feathers, and birds with teeth. Hell, you can still get teeth out of chicken cells.
The fossil record, spotty as it is, is up to its ass in "transitional forms." Even more so when later forms of life, where DNA can still be harvested, are examined at the genetic level.
I have no idea, and never did, where that particular anti-evolution argument came from