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In the course of the as'vamedha, a precise enumeration is given about
the number of ribs of the horse -- 34. This is distinct from the
38-ribbed Arabian horse, equus caballus. It appears that was a species
called equus sivalensis in the pliocene (see picture). Fossils have
been found of this species in the Shiwalik ranges in north India.
Could the as'va referred to in the R.gveda refer to this equus
sivalensis? And not to the equus caballus? There is pictorial evidence
of the use of onagers in Mesopotamia to draw chariots.