Keratin is a difficult to acquire resource in Ark. Difficult to acquire in large quantities, at least. It is harvested from some Dinos. Look for the ones with horns or bone plates.
In crafting Keratin, is almost always interchangable with Chitin.
Harvesting Keratin is not a fast process, and will require you to hunt specific Dinos. The two best Dinos for this in the early game are the Triceratops, Trike, and the Turtle, Carbonemys. Using a Stone Hatchet does work. For best results you should use a Metal Hatchet.
You won't always get Keratin regardless of the Dino you harvest or the tool used. To get an idea of what Dinos can yield Keratin, just look for horns or bone plates.