The bunny cactus isn't only named that way because of its rabbit-ear-like limbs. It's also a prolific reproducer. A single plant can produce dozens of seeds, and in turn it can create dozens of bunny cacti.
Now I think of it, the bunny cactus also develops little tufts of fur on its spikes, reminiscent of a rabbit's cotton tail. Damn, they really nailed the naming of this cactus.