[Tuidjy]

Not true. The heaviest horses (800+ kg) were arguably never used as warhorses.
Eastern horse archers used horses incongruous with Westerners idea of what a
full-size horse is. Chengis's people rode on horses that were under 500 kg.
Even the fully armored knights of the middle ages only used medium (by today's
standards) horses, as the heavy draft horses of the time were not agile enough.

Given that Dominions mixes time periods, we should assume that all kinds of
horses exist at the same time - from light weight Arabians to heavy Friesians.
It does not make sense that a horse archer will use or train his horse as a
battering ram.

But I wonder whether there is any point in talking realism. In Dominions, horse
archers are a joke, while in Earth's history, they were the bane of everyone and
everything, until the advent of firearms. Yeah, armchair generals will tell you
that Alexander somehow defeated them, and that a combination of pikes and longbows
would have crushed them... whatever.

Speaking purely from a balance perspective, horse archers either should be
drastically reduced in price, or get a 'skirmish' command - advance until in
range, then retreat from closest enemy while reserving points for a shot.
It makes no sense to give them melee abilities. They would never engage, for
example, heavy infantry.