Let's think about this logically.

Look at the Roaring Knight. The only defining characteristic we have to go on is horns. This unfortunately leaves a lot of characters as possibilities. Here is the list:

1. Dess (presumably would have antlers)
2. Rudy
3. Carol
4. Asgore
5. Asriel (presumably would have antlers)

This objectively is the only realistic list of characters the Roaring Knight could be.