I found a video that covers MX vs ATV Games from 2005-2018, so I thought I would try to fill in some of the PC / MX games that came before that too.
https://en.wikipedia.org/wiki/Category:Motorcycle_video_games
The game that started it all for me was...
Motocross Madness 1 - 1998
https://en.wikipedia.org/wiki/Motocross_Madness_(1998_video_game)
https://www.youtube.com/results?search_query=motocross+madness+1
Motocross Madness 2 - 2000
https://en.wikipedia.org/wiki/Motocross_Madness_2
https://www.youtube.com/results?search_query=motocross+madness+2
Another game showed up somewhere in this time with kinda' weird gyroscope physics and custom tracks too, but I can't remember the name (help?)! I thought it was MX Mania or Motocross Maniacs?
There was a long dry spell... until...
MTX Mototrax - 2004
https://en.wikipedia.org/wiki/MTX_Mototrax
https://www.youtube.com/results?search_query=MTX+Mototrax
MX vs. ATV Games - 2005-2018
https://youtu.be/AI0vrLd9QEo
We have had several other good games along the way - add your favorites below!
MXSIM - 2007
http://mxsimulator.com
https://www.youtube.com/results?search_query=mx+simulator
MXGP - 2014-2018
https://en.wikipedia.org/wiki/MXGP_The_Official_Motocross_Videogame
https://www.youtube.com/results?search_query=mxgp+the+official+motocross+videogame
The game between MCM 1 and MCM 2 was Extreme Biker/MX 2000. I believe one was the US name and the other was used in other countries.
Somehow that morphed into MX Mania. I don't remember if that was just a repackaging or an update.
Ah-ha! That was it - thanks DirtTwister.
So interesting to go through the history in chronological order and see the changes/new features like this. Motocross Madness 2 will always be my favorite.