Isn’t bechamel usually made with butter?

Yes, but so is white gravy, usually. The white gravy served with fried foods (like fried chicken, chicken fried steak) tend to be made with butter, but the white gravy served at breakfast (like sausage gravy on biscuits) tends to use excess sausage fat.

But if you go and make bechamel with a roux made from rendered animal fat instead of butter, it would still be a bechamel.