Simplest way is to take a bunch of minoblood wine. You can purchase it from the gate guard minotaurs in the foothills.
You can also try taking wyld wine and save scumming until you get the horsecock wyld mark. 1/15 chance iirc.
The last way I can think of is to beat the corrupt minotaur band in the valley and fuck the leader's ass a bunch. Eventually it will force TF your cock into a horse cock.