Here's a link to the information on Premarin, including its source.
http://www.project-aware.org/Managin..._Opinion.shtml

Here's news on a synthetic Premarin, posted by a Horse Advocate group!
http://www.equineadvocates.com/cenestin.html

It's called Cenestin, and was approved by the FDA in 1999.

Here's the list of results I got on Google when I did a search on "Cenestin":
http://www.google.ca/search?hl=en&q=...G=Search&meta=