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If i understand your question correctly, i heard this:- Two male 'zagal' is actually Chingis`s two horses (so its name for those horses)
I didn`t find it in secret history book, i`ll post picture of where i read this, later...
I didn`t know that horse was called Zust bor (zust brown)
Edit: Here is the photo (and another image)
http://www.mongolforums.com/uploads/1128166272/gallery_1_1_1 03054.jpg
http://www.mongolforums.com/uploads/1128166272/gallery_1_1_2 00800.jpg