Ivermectin is a general antiparasitic drug used in dogs, horses, sheep, cows, reptiles, and humans. It's on the WHO's list of essential medicines, and it was the 341th most commonly prescribed medication in the US. We use it for lice, scabies, whipworm, etc.

These people use horse paste because it's available off the shelf at Tractor Supply, whereas to get ivermectin for humans requires a prescription from a doctor. Which they're not gonna get, because it doesn't work for covid.

And they shouldn't get it, because we don't want to overuse it and end up with ivermectin resistant parasites. Hell, I wouldn't be opposed to ivermectin for horses requiring a vet prescription and doing fecal egg counts so we don't end up with resistant strongloides.