That's not exactly true, it's not necessarily horse-specific but there are variations between human and animal versions:

> Animal drugs, on the other hand, are often highly concentrated—especially those used in large animals such as horses and cows—and high doses of those drugs could be highly toxic in humans.

> In addition, animal drugs often have inactive ingredients not evaluated for use in human medicine, and FDA officials don’t know how those ingredients affect drug absorption in a human body, FDA information states.

https://www.avma.org/javma-news/2021-10-01/people-ingesting-...

It looks like the source of your mistake is missing that while the active ingredient is the same, the formulations differ both due to size and variation in how the body handles the drug. For example, differences in bioavailability or the liver’s effectiveness breaking it down affect the choices about dosage and what other compounds are used to encourage uptake, make absorption faster or slower, etc. This is why you’ll often see species-specific warnings on veterinary medication since they don’t want you assuming that you can give one of your pets the same thing you got for a different one without checking.

Veterinary drugs are separate products with a different FDA approval pipeline.

> FYI there is no horse version of drugs.

By "horse version" I don't mean a different chemical active ingredient. I mean packaging, dosage, etc. E.g. for animals Ivermectin is often in powder form so it can mixed with food, while for humans it's usually a pill. That means the human version is not approved for lifestock and the lifestock version is not approved for humans. They are two different drugs in that sense.

That's definitely not true about antibiotics.