D&D 5E Best low level monsters to freak the hell out of a low-mid level party?

[Herobizkit]

One fun tactic I like to use is "trollbolds" - kobolds with regeneration. I chased a whole party with the same pair of kobolds for a whole adventure once. They go down in one hit, PC's assume victory, off and away... and woe to those who cut their heads off...

 

[aramis erak]

I like Kobolds built from other templates as a wakeup-call. A kobold acolyte can be REALLY scary. A Kobold knight, mage or priest is worse. I've yet to use a kobold assassin. But I have thought about it. (Use the template with the DMG appendix modifications for race.)

Other similar hominid nastiness likewise.

 

[Shiroiken]

Cockatrice and Basilisk

 

[pogre]

Not creepy, but a squad of melee hobgoblins will rip you to shreds if you underestimate them. Well-armoured, disciplined and they have Martial Advantage.

To build on this a little bit. If you use miniatures, throw out some unfamiliar miniatures to represent a well known monster to throw the party a curve. I used a bunch of GW beastmen to represent hobgoblins in my campaign. Not knowing exactly what they were threw my players for a loop. The hobgoblins martial advantage made the beastmen very dangerous indeed.

 

[Erechel]

Almost everyone has great ideas here, but I say that the red staring squirrels guy and the old ladies one are my favourites yet. But Ghouls are a solid option to freak the hell out of the players. I don't want only a mechanical monster, but a monster that annoys, unnerve and disturbe the characers. I once made a group of child running zombies with magic mouths screaming "mommy" or "save us from the bad monster", and one in four explode when hitted (this was not an idea of mine, as I gently stole it from an old Internet article). Scary as , low level, but somewhat effective enough in combat to pose a threat to almost any character.
I want to freak both the characters and the players, and give them the best fight in his lifes so... feel free to share your ideas

 

[iserith]

I want to freak both the characters and the players, and give them the best fight in his lifes so... feel free to share your ideas

Here's an encounter I designed for 5th-level characters that involves rust monsters (with maps and everything).