All of the dragons in C&T, along with the lesser drakes and wyverns, have horn attacks. In prepping for my party to meet some wyverns, I was looking at this, and the pictures of the dragons in C&T and anywhere else I could find them. For those pictures of dragons with horns, they point backward on the head and would require a really contorted maneuver to attack in any way that would allow one to stab with them.
Other creatures with horn attacks include bulls, antelope, elk, rhinos, narwhals, walrus, triceratops, unicorn, and minotaur, along with a couple of others. Among those, the horns are forward-pointing and only narwhals and minotaurs get a bite attack - after or instead of, never with the horn attack. These make sense, but backswept horns, not so much.
If you've used the horn attack, how have you visualized it, or described the strike to the players? A back-sweep of the head? The horn on the nose like a rhino, stabbing? Or have you skipped it and possibly given the creature a replacement attack?