Examples of animals that co-opt toxins?
Glaucus atlanticus consumes and reuses the nematocysts of jellyfish siphonophores. Perhaps not quite what you were looking for, as the animal doesn't simply concentrate the toxin, but actually co-opts entire stinging cells.

Further to your mention of dinoflagellate toxins being concentrated by fish, I seem to recall reading something about some toxins being modified in vivo into more toxic derivatives, but cannot find any mention of it.