Recently, Hubz posted about 4 Hudson games his crew preserved here. I'd never seen the website before, but I'm so glad he highlighted it. Great historical stuff.

There was an article about the 16-bit Batman and Robin game and the development team involved. It got me curious about the game and the differences in the releases. It is single-player for SNES, but co-op for Sega Genesis.

What other games are single player vs. co-op, depending on the system?

Thanks again to Hubz.
https://www.gamingalexandria.com/

Double Dragon for the SMS is two-player co-op as opposed to the NES release.

Tons of games lose their co-op modes when moving from the arcade to console, like Bad Dudes and P.O.W. on the NES, or MERCs on the Genesis.

Golden Axe is single player on Sega CD

Golden Axe is single-player on Master System too (with co-op on Genesis, obviously). Altered Beast is another one that has co-op on Genesis but not Master System. Sonic 2 as well, but it's close to being a completely different game on each of the two systems.

Alien Syndrome has co-op on the NES, but not on the Master System.

Another one: Final Fight (single-player on SNES, co-op on Sega CD).

Streets of rage comes to mind.

Chiki Chiki Boys (single-player on Genesis/Mega Drive, co-op on PC Engine)