Over the past fifteen-plus years, the prequel trilogy has garnered a lot of love compared to when its films were first released. A combination of those who grew up with the movies being able to voice their opinions and time healing wounds, many, many fans now appreciate and love the story of the prequels, especially as things like The Clone Wars are incorporated in.
However, these movies are far from perfect, having a variety of technical issues across them. Many fans have shown disdain for the CGI, which is used a lot in the trilogy for the various species that inhabit the Star Wars Galaxy. Whether they are hurt by CGI or actually look really good, though, the trilogy does provide a plethora of alien species, some of whom induce fear into audiences.
Clawdite
Appearing in Attack Of The Clones, the fan's first exposure to the Clawdite was through Zam Wessell, a bounty hunter hired by Jango Fett. While definitely not terrifying or as scary as others, there definitely is a level of fear that comes with the Clawdite.
This is especially true for younger viewers. One moment they are looking at a human bounty hunter, the next, she becomes a reptile-like species, her true form. Couple that with the knowledge that she is a bounty hunter who is, presumably, pretty lethal in her career before this movie, and her fear stock rises.
Kaleesh
Audiences only get treated to one Kaleesh in the prequel trilogy, and he is a bit of a cheat for this list since he is mostly cybernetics when we meet him. Nevertheless, General Grievous is still an undoubtedly terrifying figure, and regular Kaleesh are not much better.
With his slit, evil-looking eyes, visible heart, and lungs covered by metal plating, giant height, and robotic movement, Grievous is terrifying for most children who grow up with the prequels. Regular Kaleesh are bat-esque in their facial features (kind of like Grievous) with red skin and walked on their toes. They also possessed the same eyes and flammable organs that Grievous had.
Dathomirian/Zabrak
Sticking with the secondary villains of the trilogy, before Grievous got introduced to fans, we met Darth Maul - later just Maul - who has a short time in live-screen but is also involved in one of Star Wars' greatest battles.
The type of fear Maul brings out is more intimidation. He is a super athlete, with dark robes, red skin, horns, full-body tattoos, and a double-sided red lightsaber for good measure. While all of this adds to his cool factor, from an objective point of view, that is a scary guy who you would avoid on the street. Being the only representative of the Dathomirian sub-species of Zabrak in the trilogy, this entry basically relies on Maul, but the other native Dathomirians are just as scary.
Pau'an
The most famous example of a Pau'an in the Star Wars canon is, of course, the Grand Inquisitor, a former Jedi Temple Guard turned head Jedi hunter under Darth Vader. But, he is not the only on-screen Pau'an fans have seen.
In Revenge Of The Sith, Obi-Wan is greeted by Tion Medon, Port Administrator of Pau City and Pau'an. The species is visually striking with long limbs, gray skin, jagged teeth and nails, and dark eyes with red rims; everything about them is intimidating, which played in the Grand Inquisitor's favor.
Reek
The Geonosis Petraki arena battle in Attack Of The Clones is perhaps the highlight of the movie and also home to three of the scariest alien creatures in the trilogy, starting off with the Reek.
The Reek's CGI was the worst of the three, definitely knocking off a bit of the fear factor. Ignoring that, though, it is a rhino-esque beast thirsty to charge into people and cause havoc. Naturally, Reek's are actually herbivores, but in its state of capture on Geonosis, this Reek is bloodthirsty and dangerous.
Acklay
Moving onto another animal of the arena, specifically, the one Obi-Wan mostly does battle with, the Acklay is far more terrifying than the Reek as for one, the CGI is better, and for two, it does not represent a rhino; it instead is like a giant, reptilian mantis-esque creature.
It is a typical intimidating monster-like being with its claws, hard shell-like exterior, and razor-sharp teeth. While it will not cause many adults to cower in fear, nor people rewatching the movie, for first-timers and younglings, the Acklay can be an intimidating presence.
Nexu
By far, the most frightening of the monsters present in the Geonosians Petranaki arena is the Nexu. Not only is it the scariest looking creature, but it is also the only one of the three that causes any real damage, clawing at Padmé's back.
The Nexu is a feline creature, immediately putting a sense of fear in those who have trust issues with cats. Even ignoring that, the Nexu has four eyes, two of which can utilize infrared vision, scarily sharp teeth, insane claws, and it moves at an incredible pace with some great climbing ability, all of which add to its fear-factor.
Colo Claw Fish
Jumping back to The Phantom Menace, there were certainly some formidable foes that the heroes had to fend off, even ignoring the Dathomirian Sith Lord, two of which came underwater on their travel from Otoh Gunga, one of which being the Colo claw fish.
The Colo claw fish had a huge eel-shaped body which accompanied an alligator-esque head with rows of razor-sharp teeth as well as mandibles and a set of venomous fangs. It is an unnerving creature, and one look at it conveys that perfectly.
Kouhun
Moving onto a species of creature that is the polar opposite of Colo, two Kouhun's are seen in Attack Of The Clones as the anthropods Zam Wessell uses to try and assassinate Padmé in her sleep.
The combination of their creepy-crawly look and their lethal venom makes them a hair-raising creature. For those who have no issues with bugs or small creatures of that nature, the Kouhun's are not that scary, but they are horrifying little guys for those on the other side.
Sando Aqua Monster
Earlier on, the Colo claw fish was talked about as one of two underwater beings the heroes encounter on their travels, but despite its huge size, Colo had no chance against the inevitable bigger fish, the Sando aqua monster.
Growing up to two-hundred meters long, the Sando aqua monsters are colossuses with muscular limbs, hands capable of gripping, and teeth capable of ripping their prey apart with a bite. It is undoubtedly the species that would leave you the most horror-struck.