Baby tooth on Flickr.
This very small snapping turtle hatchling still has its egg tooth. You need to look closely.
Turtles, crocodilians, and birds have an analogous horny structure that breaks the shell of the egg to allow them to escape from the egg at hatching.
The only mammals to hatch from eggs, the duck-billed platypus and the echidna, also develop an egg tooth before birth.