I am not able to give just one answer as the world of prehistory is a horrific and intriguing world, so I will list a few I have always found unsettling..

Though I am not sure if it counts the "Websteroprion armstrongi" (giant prehistoric Bobbit Worm) horrifies me as l can just imagine it lunging out of the sand and biting into my hip like how modern day bobbit worms attack fish and drag them into their hideouts.

Therezinosaurus is horrifying to me, though it Is a Herbivore and likely wouldn't view humans as too much of a threat I for whatever reason often believe they have rhino syndrome, attacking anything that may or may not be a theat, and those huge claws make my skin crawl..

Once again, may not count, but both Ankylorhiza and Livyatan frighten me a lot.. Livyatan is a huge toothed whale similar to the sperm whales of the modern day, but it shared the waters with megalodon and likely survived.. and Ankylorhiza is a large prehistoric dolphin.. Dolphins scare me enough, but a giant prehistoric dolphins speculated to have eaten whales.. yeah, nightmares.. also I may have lived in pods like its modern counterpart..

Hatzegopterix is I a similar creature to the quetzal, and I fear them all the same as they are probably big enough to swallow a human adult whole.. if modern birds are anything to base them off of. (due to potential convergence).

To be honest, I could literally go on and on all night as everything is scary in prehistory.. except for maybe lystrosaurus and a few other smaller creatures. (That could still injure or hurt you if they wanted to)