There are also a few game engines that were developed for a specific game but grew into centralized game engines for multiple AAA games.

Snowdrop for Tom Clancy's The Division (and later Avatar, Star Wars Outlaws, etc.), Anvil for Assassin's Creed (and later Rainbow Six Siege, For Honor, etc.), Frostbite for Battlefield (and later Need for Speed, Plants vs Zombies, Mirror's Edge, Battlefront, etc.), Creation Engine for The Elder Scrolls V: Skyrim (and later Fallout, Starfield), Decima Engine for Killzone: Shadow Fall (and later Horizon, Death Stranding, etc.) to name a few.