Belostomatid bugs have got to be one of the eldritch-est.
They are large, active aquatic predators that kill by injecting their toxic saliva into their prey. Or into any humans stupid enough to pick one up—not that I’ve tried, but their bite is supposed to be pretty painful. I occasionally catch these guys in stickleback traps. (Below is a picture of one such bug right after I released it.) Although they’re aquatic, they breathe air, and if they’re caught in a trap for too long they can actually drown.
Just how horrifying are they? This horrifying.
(Just a note on the taxonomy in the above article. All members of the family Belostomatidae are giant diving bugs; the culprit in the article belonged to the subfamily Lethocerinae. Belostomatids belong to the order Hemiptera, also known as “true bugs”, which includes cicadas and aphids.)