This is actually not a Breyer, but sculpted by Don Manning of Nosco Plastics. Breyer copied the sculpture for the Modernistic Doe and Buck molds. Added to this site to show the difference between this original model and Breyer's version. Estimated made in the 1950's.
For an excellent history lesson on these models, and creator Don Manning, check out Kirsten Wellman's blog post: http://modelhorsecollectibility.blogspot.com/2018/03/.