Cook ground bison and onion over medium heat in a large skillet until ground bison is brown and onion is tender. Drain off fat. Stir in cumin, salt, coriander, cayenne pepper, and water. Cook and stir for 2 minutes.
Whisk eggs and milk in a medium bowl. Add to ground bison mixture in skillet. As the mixture sets, run a spatula around the edge of the skillet, lifting egg mixture so uncooked portion flows underneath. Continue cooking and lifting for 2 minutes or until egg mixture is almost set.
Evenly divide ground bison mixture between taco shells. Sprinkle with cheese. Serve with Pineapple Salsa.
Pineapple Salsa: Stir together pineapple, bell pepper, onion, cilantro, lime juice, jalapeno pepper (if using), and salt in a medium bowl. Refrigerate for up to 24 hours.