Video game bootlegs are unlicensed video games produced in breach of someone else's copyright. There are some examples of them:
Nintendo Entertainment System
7 Grand Dad (1992) - A ROM hack of The Flintstones: The Rescue of Dino and Hoppy (NES) where Fred Flintstone's head in his in-game sprite was edited to look like Mario's head. Interestingly, this game is a meme. The meme started at https://www.youtube.com/watch?v=lRgl6K1Qecg, by the way.
Donkey Kong Country 4 (1997) - A remake of Donkey Kong Country (SNES, 1994) made by Hummer Team.
Sonic the Hedgehog (year of release unknown, but it's a ROM hack of bootleg game Somari (NES, 1994)) - A remake of Sonic The Hedgehog (Sega Genesis/Mega Drive, 1991). On November 20th 2016, the jabu created an improvement of the bootleg created Sonic the Hedgehog (NES) - Improvement + Tracks. It was then further improved upon by Ti and re-released on December 1st 2016. To find out what improvements the improvements have made, I suggest you go to https://info.sonicretro.org/Sonic_the_Hedgehog_(NES)_-_Improvement_Plus_Tracks.
Super Mario World (1995) - A remake of Super Mario World (SNES) made by Hummer Team.
Sega Genesis/Mega Drive
Super Donkey Kong '99 (1998) - A game based on the Donkey Kong Country SNES games with some levels of Donkey Kong Country and a few levels of Donkey Kong Country 3. To find out more information about the game, you can go to http://bootleggames.wikia.com/wiki/Super_Donkey_Kong_'99.
Super Nintendo Entertainment System
Sonic the Hedgehog (1996) - A ROM hack of Speedy Gonzales: Los Gatos Bandidos (SNES, 1995) by Peruvian scene organization Twin Eagles Group.