You are thinking of Rotor. FWIW, I also feel as if Portable.NET--which was rebranded at some point to DotGNU when I think it was even donated to the FSF--had predated Mono in functioning?
The Mono website has an archive of an old mailing list post which at the time talks about even-older origin of the project. It is (of course) heavily biased for Mono, and hilariously gives me an awkward shout out ;P.
So it ran on Windows, FreeBSD and Mac OS X making it the first non-Windows implementation of .Net, but it didn't run on Linux. It also had a fairly useless licence, so Mono was separate.
The Mono website has an archive of an old mailing list post which at the time talks about even-older origin of the project. It is (of course) heavily biased for Mono, and hilariously gives me an awkward shout out ;P.
https://www.mono-project.com/archived/mailpostearlystory/