There´s no such thing as "zoo porn". Animal porn or bestiality porn, but no "zoo porn".

As you might know, I´m not interested in anything else but mares, so I won´t be the best source when it comes to other species or male horses. But I´m experienced with mares and can tell you there´s no such thing as a special "orgasm grimace". Mares just use their "everyday" expressions, even during and after sex. For example, a common expression of feeling good, relaxed and satisfied is "chewing the air" and licking lips. Every mare I slept with did that during intercourse. With the smallest mare I ever slept with, a Cremello mare named Lea, I even had a wonderful experience: she turned her head around during intercourse, looked at me and started chewing, so I leaned to her right side, she stretched her neck so I could touch her cheek with my hand ...and with some more stretching from me, our tongues touched while she was licking her lips. We stood for some time, licking each other´s tongues while I was in her...never experienced anything like this again with any mare. Lea must´ve been the most flexible horse I´ve ever seen.

If you really want to know whether a mare enjoys "it", you will find more clues in behaviour than mimics. A mare with her hind legs spread, a mare chewing and turning her head around, a mare who does lift her tail straight up to the point of her tail hairs lying on her back...all of that are signs of enjoyment. Some of the mares I slept with literally grunted, sounding like "ooooohhhfff!" during climax, some of them never made a sound, but I could feel them coming by the intravaginal muscular contractions that differ from the largely misunderstood "mare twitching" many "zoos" are so certain of as a display of climax. But to my knowledge, all mares show orgasm by arching their backs, literally sitting on your lap. My Hannover mare sometimes farted during climax...horse farts usually are silent, but when you are standing behind your mare, with her pressing back- and downwards at the same time, your belly firmly pressed to her anus, horse farts are GODDAMN LOUD. ;) Imagine pressing your hand against your mouth and blowing out air with force...;) The first time this happened, I almost fell from my plastic pedestal laughing...

Generally spoken, the variances in mares´ orgasmic behaviour aren´t big, but they all differ in details. Some grunt during climax, some don´t. Some are more actively participating, like the Haflinger mare I slept with, a literal sex monster, some others are more passive. Some will look at you for a long time with their heads turned around, some will take a quick peek "over the shoulder" and then lower their heads, ears pointed sideways in relaxation, chewing the air. Some like being licked, some don´t like it at all, but were very much into being penetrated. Some like their anus "massaged" during intercourse, some don´t. Some like their flanks scratched, some like it when I scratched her backs and hips, some liked to be carefully bitten in their backs, some like the base of their tails, the hairless backside of the tail, scratched or caressed. Some mares like it when you play with their teats/udder as foreplay, some others absolutely don´t like it.

Hope that answers some of your questions regarding mares.