It's also used on dogs, but mentioning that it works on horses is more impressive since horses are known as very large strong animals. I think that's the reason it's often called a "horse tranquilizer", to emphasize that it's serious stuff.
I mean, the article makes clear that Dr. Harry is an anesthesiologist and that anesthesiology is a very serious discipline, so I don't think the author meant to suggest they were carelessly administering medication meant for animals instead of humans.
The irony is that it's actually one of the safest
(and cheapest) tranquilizers and that's why it's used on large animals. It's hard to weigh a horse, so you want something that doesn't require a very precise dose.
I mean, the article makes clear that Dr. Harry is an anesthesiologist and that anesthesiology is a very serious discipline, so I don't think the author meant to suggest they were carelessly administering medication meant for animals instead of humans.