I do know that the splint bones are vestigial (reduced non functional toes in this case) , in odd and even toed ungulates. Is there any evolutionary reason of why is it so? What were the conditions that lead to it?
One easy way to make an animal faster (especially if it is larger than a housecat), is to make the ends of the limbs lighter, the more mass there is near the end of a limb the more energy it takes to move it and the slower you can swing it, that translates into moving slower because you cannot get the limb back into position for the next step fast enough.
Because of this reduction in the mass and number of toes in lower limb is an adaptation we see over and over again in animals focusing on running speed. We see it in dinosaurs, we see it in ungulates and artiodactyls. We see five toes reduced to three then eventually one toe, or sometimes we see a reduction from four toes to two paired toes instead.