Ooh, what a discovery!
In the default avatar menu, you can find two horses; the girl with a bow and arrow (Rhiannon) and the boy (Thomas) with a sword next to her, both have horses. They both can be used with a mesh avatar as long as you turn off your own AO and add the riding ao matching your gender in one or the other avatar.
No more need to not go horse riding because your friend doesn’t own a horse. 😀
How cool.
That’s all.