Correct me if I'm wrong, but aren't the others just merely clones of the original Bitcoin, using the same source code? I would still call Bitcoin the only currency, as it's the base they all share.
In this case, multiple currencies can share the same code base, in the same way The pound and the dollar share most of th same rules Of fiat currency.
It seems like there's a bit of terminology overloading going on. You have Bitcoin the protocol, which all these things are using, but you also have Bitcoin the currency, which is separate from the other currencies using the same protocol but with different networks (or block chains or whatever you call it).