If you are looking for someone in Europe, there is a company in Holland, they made all castings and moulds for Josine Vingerling for example, so they have experiance with casting horse models
It is called De Tinnen Roos: http://www.detinnenroos.nl/nl/ID/562
I guess you would need contact them directly about details because they don't show this option in their regular offer.
Also, you can try asking at FormX (https://www.formx.nl) they sell products to many companies, perhaps they know someone?
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Anna
Horse and Bird studio - Horse sculptures
My model horse collection