testosterone is a particular chemical compound, and the animal or human that it is injected into has no bearing on the product. testosterone is testosterone. The only variations are the esters (prop, cyp, enth, etc)
But yea, there is no such thing as 'pure horse testosterone'. That's hilarious.
People have been procuring steroids from vets for decades. It's not unusual. Tren and EQ are essentially 'horse steroids'. They were both developed for horses, and we use them, just in different amounts, as we respond differently to the compounds.