They're parasitic creatures that attach themselves to humans and feed on a natural energy field that enables all manner of psychic abilities (if allowed to grow, instead of being used as food).
Perhaps the most terrifying aspect of these creatures is that they aren't, strictly speaking, physical - they're gigantic muddy-looking shadows that hop around like a 100-ton kangaroo on steroids.
I don't know how seriously to take ol' Carlos (who was high on mescaline for most of his 'experiences') but his concept produced a neat sketch.
