Is it fair that people called it a horse dewormer when it is safe for use in humans? No.
But, there were issues with dosage and not taking it as it would have been prescribed. People were often taking a dose formulated for a horse and overdosing.
https://scitechdaily.com/ivermectin-can-people-take-a-drug-for-horses-and-cows-to-treat-a-deadly-virus/
"For one thing, animal drugs are often highly concentrated because they are used for large animals like horses and cows, which weigh a lot more than we do—a ton or more. Such high doses can be highly toxic in humans. Moreover, the FDA reviews drugs not just for safety and effectiveness of the active ingredients, but also for the inactive ingredients. Many inactive ingredients found in products for animals aren’t evaluated for use in people. Or they are included in much greater quantity than those used in people. In some cases, we don’t know how those inactive ingredients will affect how ivermectin is absorbed in the human body."
https://www.fda.gov/consumers/consumer-updates/why-you-should-not-use-ivermectin-treat-or-prevent-covid-19