I'd be only moderately surprised if it's a new species. And the surprise doesn't come from the fact that I'd think humans would have discovered it earlier, but rather that one would wash up shore.
Science has taught us more about the entire universe than our own oceans. Some depths are impossible to reach, even with a machine using today's technology. It's quite possible that undiscovered creatures dwell in the depths. That's what's tricky though... if something lived
that far under the sea, how would it ever surface?
But, that'd be a reason why we've only seen two.
P.S.
"Octopus Giganteus", talk about original naming.