Rhiannon is also the name of one of celtic pantheo's-- she is the godess of horses, (and poetry, and other things)

the horse was held to be an 'familiar'--a shape a god would take when visiting earth, and all of the people of the british isle dislike the idea of eating horse meat (you don't eat gods)--a cultural remnant of old believes.

ancient irish lore, has it the that the O'Connor king's had to have sex with a mare before assending to the throne. (i don't know if queens had to undergo a similar, but more grizzly encounter with a stallion.)

in the book A Man in Full there is a very graphic description of a stallion being put to stud. the mare in question didn't have an easy time of it.