From what I understand:

• Paste does not work on cows because it will not survive they stomachs. You must inject them.
• Horses used to be injected with Ivermectin, but switched to oral because of risks of abscess and it hurts. People care about horses more than food.
• Can give injected medication orally to horses but it is non optimal. Being more of a liquid, it is easier for horse to spit out.

Some thoughts from actual horse owners before the pandemic: https://www.pedigreequery.com/forum/viewtopic.php?f=8&t=31332

Since the liquid is meant to be injected, each component must be purer. My biggest complaint with the paste is it has 3 proprietary components. I don't know what they are. I believe one of them is an aluminum based preservative. I try and avoid taking aluminum.