We've always known this as Aporocactus flagelliformis, but I understand it's now considered a Disocactus. It's hard to keep up with the lingo sometimes. But that's OK, since either name is correct. After all, that's what synonym means.
I've often said that it's good to have a magnifying glass at hand (or a close-up lens) in a nursery. That nice, almost fuzzy appearing, "rat-tail cactus" isn't fuzzy after all. It's wearing spurs.