One of the most common estrogens for HRT until recently was Premarin.
Premarin got its name because it comes from PREgnant MARes' urINe.
So, if you're a trans pony girl with a breeding and piss kink, there's a regimen for that!
@LilahTovMoon
perfectly normal scientists were gatherine the urine of pregnant mares compressing it into pills and selling to members of the public to consume, then you came along amd you had to make weird didn't you!