You can get the Horse64 development kit on the collaboration platform “Codeberg”:
If you don't know which variant to get, you probably want this one:
Latest SDK download for Microsoft Windows (x64) (From Codeberg)
If you're adventurous and curious, raw source is here.
Horse64 is open-source with BSD-2-Clause or Apache 2 terms, click for details (the only exception being the official branding, which you can swap out). Also, third-party things have their own licensing.
Go here.