<Oldest horned dinosaur in the world would perhaps be _Proceratosaurus_, not a
ceratopian at all, or _Ceratosaurus_. Do nasal crests count as horns?>
If we would go so far to call the horned dinosaurs every dinosaur that has a horn/s, bump or spike on its head we would have to expand the list signifficantly. We better stick to the "established" term - meaning: ceratopsians with horns.
Beri's Dinosaur World