Xamarin is the primary maintainer and commercial sponsor of Mono. Xamarin - a subsidiary of Microsoft - is a company founded by the engineers who created Mono.
However, the differences between public MonoMac and Rhino for Mac’s MonoMac are inconsequential. That means Rhino for Mac is not using the public Mono, nor premium Xamarin.Mac. RhinoCommon on Mac is executed through an embedded, custom, Mono framework.
