There are probably numerous large-ish species still undiscovered that live on the deep ocean floor… animals that never come up in the water column and when they die they remain miles beneath the surface.
Things like whales moves around and can float and get washed up when they die, even if they live their lives out of view of us. Worms and other scavengers that live in deep sea sediments? I think it’s much less likely they’ll wash up for us to discover.
And whales, specifically, being mammals, do have to surface regularly in order to breathe, thus increasing the likelihood that a human would see them, at least on occasion.
The deep ocean floor has very little oxygen and very little food—not a very likely place to support anything too big.
(It is like SF showing extremely dry, barren deserts with giant predators that it could not support. Sandworms? Sarlaccs? Nope.)
Some species of whale are rather partial to squid. Some of these are very big indeed and can put up a fight. I guess they don’t spend all their time in the abyss, but move up and down the water column to find food.
It is not inconceivable that there are some undiscovered species of squid down there. The oceans are deep and vast and there is much to discover.
Some species of tree kangaroo native to New Guinea were only discovered in the 1990s. Certainly seems that there could be more out there.
If you did, you certainly wouldn’t now, if she sees that comment. 
Sandworms aren’t predators, they’re plankton feeders. And possibly lithovores (especially if you consider the Dune Encyclopedia canon, as I do, but even if you don’t, there are other hints at inorganic nutrition in the books).
They don’t attack larger prey like humans and spice harvesters for nutrition, but out of territoriality. I wouldn’t call them predators, any more than hippos are.
And as to the deep ocean floor - there are some sites with better food prospects, like hydrothermal vents. Oxygen is the real limiting factor.
Paleo-duck?