Dog dick for the feel. Nothing feels so good by virtue of gliding so effortlessly. Nearly frictionless. No drag on an outer epidermal layer. That raw, bare flesh tissue is a perfectly compatible, like-to-like match for our insides. Sure the size is underwhelming next to a horse, but that's feasibility: of being fucked by something that won't kill you (peritonitis is a thing; just ask Kenny Pinyan) nor take ages of training til you're risking shitting in a colostomy bag for the rest of your life. Points won for being a lover you can take to bed (literally) too. Can't spoon an equine.
But horse? Horse or jack donk for a phallus big enough you can still hug it to your chest and kiss it while he's mounted up all the way behind you thrusting it between your thighs. Yeah, the thing still reaches your face if you're angled right and you don't have a paunch in his way. Not even boars can boast that. Oh did I mention the wonderfully redolent fossa smell of gamey, fermented-to-perfection equine maleness? Nothing smells quite so wonderful as horse dickhead (tho boar smeg is a close second). Ooooh, that hidden truffle of heady, lusty, olfactory high. Finally, horse for the love of choking on a dick that flares your jaw apart and batters the back of your throat til the tears of joy are running down. If you like that sort of thing (I didn't -- but I've learned to).
Nah. Unfair. Don't make me choose. You know I can't.