Dr Adequate writes:
It is as though you privately redefined "unicorn" to mean any animal with a single horn on its head, and then went around saying that I believe in unicorns, justifying this claim to yourself on the grounds that I do believe in rhinoceroses.
Not to take away from the heart of your analogy, which is of course, quite accurate, but Rhino's (with two exceptions, but that's really more of a "stump" than a horn) have
two horns. If you wanna talk real horns, only two have them, the black and the white. The black has two large ones, the white has a large one and a small one.
Black:
White:
Indian:
(Stumpy 1)
Javan:
(Stumpy 2)
Sumatran:
Why do I know this? I didn't. In fact, I thought there were only two species of Rhino, black and white, but I was fairly certain they had two horns. And they do. But to my surprise there were three more species of Rhino, and two had a single bony growth on it's snout, the others all have two.
Anyway "rhinoceroses" can't be said in general to have only a single horn.