What mammal has a bony armor and can roll into a ball?

The mammal with bony armor that can roll into a ball is the armadillo.

Armadillos are New World mammals whose backs are protected by ossified plates called scutes. These bony structures are covered by keratin, the same durable protein found in human fingernails. Flexible bands connect sections of the armor, allowing the animal to bend while still protecting much of its body from predators.

The complete-ball defense is especially associated with the Brazilian and southern three-banded armadillos, the only armadillo species able to close themselves into a nearly sealed ball. They tuck in their head, tail, legs, and ears, leaving the armored exterior exposed. Many other armadillo species have too many rigid plates to roll up completely; they may dig, flee, or curl partially instead.

Armadillos are often confused with pangolins, which also have protective coverings and can curl up. Pangolins, however, are covered in overlapping keratin scales and are native to Africa and Asia. Armadillos are native to the Americas and are skilled diggers, using strong claws to excavate burrows and find insects.