It depends on how excited you get the horse, how tight are your thighs in the saddle, what are you saying in the horses ear, have you tickled his testicles before you mounted him, foreplay is important to a horse.
Larger horses are probably more than 18". Horse sex is brutal. The stallion can smell the mare that's in heat and starts 'courting' her, she many times runs and bucks. He proceeds and tries to stick his muzzle in her *****. His **** has 'dropped' out of it's sheath 8 or 10" and is flopping as he chases. She kicks and runs again. Finally she settles down to the inevitable and her tail comes up and to the side an he shoves his muzzle into her now somewhat gaping *****. She many times ****** on his muzzle. She widens her stance and braces herself, ***** gaping, tail to the side. The hormone laden **** and ***** juice causes his **** to reach full hardness almost instantly and is parallel to his stomach. He mounts her and starts shoving at her ***** violently. His mushroom head is now huge as it finds her slot and he thrusts hard to penetrate her. He pumps hard 3 to 5 times and thrusts it full length as he pumps her full. As soon as he shoots the last of it his now deflating **** drops out and shrinks back into its sheath. She stands there for a moment, her ***** kind of spasming and leaking copious amounts of thick *****. Once she settles down and his **** is hard it's all over in about a minute.
According to Wikipedia a horse is roughly 20cm long (about 8") in flacid state but doubles in length when erect, while the glans increases 3 to 4 times. <br />
Catherine the Great of Russia is said to have while having sex with a horse and a gay man from Seattle liked horse sex and suffered an intestinal rupture during a session. He didn't go to hospital and died.
Let's say she had a reputation! She was high on shrooms at the time too.
Most horses have an erect **** that is 18" long, but that is only if they fancy you.