[LK44-40]

would even be able to cover a mare

This is horsie talk for making the beast with two backs? (Of course, I know that horsies do it dog-slyle, so to speak. I'm no ~fool~, after all!)

[Erasmus]

This is horsie talk for making the beast with two backs?

Well, in the case of horsies, it's the beast with one back, one front, and eight legs.

< ]B^)

[Jemian]

This is horsie talk for making the beast with two backs?

Yes. It refers to sexual intercourse.