I've written an amateur, hobby-level engine, described here: https://mellinoe.wordpress.com/2017/01/18/net-core-game-engi.... This was more of a personal exercise for learning and experimenting than a serious attempt at game design, so the engine isn't that advanced.

For something more professional and polished, there is Xenko, which runs on CoreCLR for some of its non-mobile targets. https://xenko.com/

Also, Unity itself doesn't really run on Mono -- it just has a scripting layer that hosts Mono on top of its C++ core. Perhaps not a useful distinction for you, but I thought I'd mention it.

(Note: I work on .NET Core at Microsoft)