You can get the Horse64 development kit on the code sharing platform “Codeberg”, here's the desktop default for Windows PCs:
SDK
for MS Windows (x64)
Download
(From Codeberg)
For other devices, check here:
Note: The original binaries are only provided via Codeberg for use by other developers. If you find them anywhere else, they were provided by people outside of the project.
Note: 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.