Heat the oven to 375°F. Lightly grease 8 cups of a standard muffin tin.
In a large bowl, whisk the egg with the salsa and taco seasoning. Stir in the oats, black beans, corn, and half of the shredded cheddar.
Add the ground beef. Mix gently with clean hands or a fork just until everything is combined. Avoid packing the mixture tightly, which can make the muffins dense.
Divide the mixture among the 8 prepared muffin cups, filling each cup evenly. Sprinkle the remaining cheddar over the tops.
Bake until the muffins are cooked through and the centers reach 160°F. Let them rest in the pan for 5 minutes, then loosen the edges and transfer the muffins to plates.