How about the platypus? It's the only living species in its genus, and it's also the only living genus member in its family.

The dugong is at least as lonely. It used to share a family with the Steller's sea cow, before that was hunted into extinction.

The narwhal isn't quite so lonely. It's the only species in it's genus, but it still shares a family with the beluga (which is also the only species in its genus).

In any case, Homo sapiens isn't unique in, er, being unique. It's just a question of how much diversity develops, and then how much of it survives.