Some creatures from mythology and fiction that do not already exist in a game are fun to develop stats for in game use. I have only created stats for a few creatures myself, and have yet to use them in my campaign, but have played in games that had them.
For me, the most memorable are creatures in my brother Robert’s game.
Stormhounds from Chronicle of Amber – I don’t have specifics, but Robert mentioned that the adults have 12 HD. The main NPC wizard has them to guard his tower and raises them. Many an adventurer has offered a lot to try to get a puppy.
The Rancor from Return of the Jedi – See Out of Character Outbursts.
Modifications of existing creatures:
Stewart war dogs 3 hd instead of 2. In my brother’s game, the hillsmen are based on Scottish clans. My character in his game, Griswald, is a Stewart and raises these dogs. They were historically (in the game) used in battle. The dogs would be lined up in front of the battle lines with collars with sharpened blades, thus they are also called “Sword Dogs”.
I read about the labors of Hercules back in February and yesterday I finally did stats for the Stymphalian birds with beaks of bronze and metallic feathers they can launch. I have not seen the movie with Dwayne Johnson/The Rock that came our earlier this year, to know if it included them.
What mythical or fictional creatures have you stated for use in you game(s)?