Prehistoric Horns are a type of Custom Mutation. The only way to get this mutation is randomly from a Mutation Pear, or by making a Custom with it. When creating a custom with Prehistoric Horns, you cannot change their color. They will always come out white.
The mutation that is now known as Prehistoric Horns was originally called Senz Horns (created for and named after Senz). At a certain point after the renaming of the mutation, they also underwent a redrawing on at least the Egyptian breed. Here is an example of the way they used to look.