Here's a link to Van Dyke's web page for lifesize horse mannikins.
http://vandykestaxidermy.resultspage.com/search?w=Horse&p=Q&ts=customI didn't realize they sell them with or without leg rods. This a
huge plus when alterations may be needed! Like I say,
any mannikin gives you a good base to start. The final quality of the mount --
any mount -- depends on the skill level of the taxidermist doing the project. If you know what you are doing, and have the abilities, you don't need a mannikin with all the "bells and whistles" and overabundance of detail to create a terrific piece.
Also, if it were me that had this opportunity again, I would contact the owner, and ask permission to measure the horse ahead of time. You never know with horses ... someting may happen to hasten it's being put down and at a time when you are not available, like late, late night, early morning, or while you are right in the middle of skinning something else. Hey, it couldn't hurt to ask.
Best of luck to you,
John.