Although actually Wyeth stopped using any pregnant mares sometime in around 2000 and now makes it entirely synthetically (at least in europe). So from an animal welfare point of view its not now any different from any other estrogen.
Its a conjugated estrogen cocktail containing about five different estrogens which in many respects is closer to normal female endocrine status than taking a single unconjugated 17beta-estradiol.
In many respects its a rather mixed bag because in terms of feminisation those of us who took it in the late 1970's and early 1980's, when it was more or less the only show in town, got pretty good results. On the down side however it contains an equine estrogen called equilin which the human liver struggles to metabolise so it wasnt very kind to the body.
Nevertheless I took it for 20 years without much ill effect, so I think (animal welfare aside) one can get a bit over worried about all this.
That said Premarin pills came in three strengths 0.0625mg (red) 1.25mg (yellow) and 2.5mg (purple) - for obvious reasons I'm not going to tell anyone what the prescribed dosage was, but they were sugar coated and oval - so not easy to mistake... So I don't think what you have there is premarin.
