I have this unit (not a TENS because it works on a different wavelength, I think, I call it a microcurrent unit):
https://therapyproducts.net/proddetail.php?prod=BV-Pro-kit
It’s expensive, but it has made huge positive differences for both my horses and myself. I bought it after watching one of my body workers use it on a couple of my horses with very obvious positive results. I most often use it on my horses at shows after big classes. I also use it at home if trying to work on a particular area (e.g. one horse gets tight through his SI area/pelvis and this gets him stretching, licking, and chewing). I can also pinpoint where a horse is tight before my vet/chiro comes out because it’s so apparent with this unit where the muscles are spasming (not that my vet needs the direction, but it’s interesting to me!).
I also use it on my back at shows. My TB in particular used to really fling his body over fences and it tended to lead to a sore back and neck for me. I started using my bestvet and stopped having to live on tylenol/ibuprofen at shows.
A couple of other points - I don’t think you need to use gel on an animal with fur. With my unit I get my guy wet and that’s enough. I also have an attachment that looks like a little hairbrush with prongs that works well on fur. When I use it on myself I use those sticky gel pads you can buy (rather than having to use gel).
But as others have mentioned, the difference between intensity of a horse versus a human is massive, and not in the way I expected. With my bestvet there are 4 or 5 setting levels with 10 levels between each of those - so 1-50 clicks, give or take. I use it on 2 clicks for my horses (and even then have to turn it down to 1 sometimes), which doesn’t even register on the first setting level. When I’m using it on a horse I can’t even feel the pulses. When I use it on myself I’m usually up at the 45-50 click level.