The man was like nine feet tall.

7967809
big horses

7967809

There are a few explanations.

1. Jaghati and a few other Primarchs rode horses in their teens, back when it was still practical for them to do so.

2. Jagahati's personal horse was just a particularly chonky steed.

3. Even modern horse breeds can be pretty big. There being a horse big enough to carry a nine-foot (super)man without too much trouble isn't too far-fetched. Especially since a lot of stuff in 40k tends to push survival of the fittest pretty far.

7967810

7967813
Fun fact: the original name for horses roughly translated to Tall Dog

1. Jaghati and a few other Primarchs rode horses in their teens, back when it was still practical for them to do so.

So given the way they shot up like bamboo, I'd imagine that was, what, five minutes

7967809
It's been thousands of years since our time. Who knows just how big horses have become in that time? Evolution or genetically engineered. Chogoris is also not Earth, so it's not a stretch to think they've evolved in any number of ways from the original Terran stock in all that time.

Long term breeding created horses big enough.

7967905
For all we know, they have horses like those of Ganondorf's