The terror bird from the game Ark. The scientific name for them is Phorusrhacids, and they went extinct less then a million years ago. Basically a terror bird is like if a bird woke up one day and decided he wanted to be a dinosaur again.
Anyhow I've gotten the best results running the model at .8 but your results may very. Use the word Terror Bird to generate the animal. Add the word saddle if you want a saddled variety, and using riding if you want an image of someone riding the terror bird. You might need to play with the prompt a bit to get a good riding image, it tends to want to use a caveman riding the terror bird regardless of what you want.