1:Skeletons) You may be wondering, "Why a skeleton?" Well, my world lore states, "When the dead are brought back unwillingly, their' metaphysical mind is usually partially shattered. As a result they are often deranged and/or insane, assuming they are more than a mindless husk. Skeletons are known to be particularly psychotic and demented, and highly unstable. The Delirium Fortress on the Isle of Solitude is a reminder of this, as the 2,000 heavily armored and armed skeletons, who are schizophrenic as well as unjustifiably racist towards all who have flesh, unleash nigh-endless tornado-like volleys of magic arrows, cross-bolts, cannonballs, and magic. If the fortress itself wasn't unfathomably durable, alongside the seemingly endless supply of magical ammunition."

An in-game example from when my players were level 6: A single vanilla skeleton with uncannily lucky rolls managed to get on fire, and grapple the druid for two rounds. He nearly died from the fire, while the skeleton screeched aloud mario-sonic fanfiction until its demise.

10 in-game days later, a gang of lederhosen-wearing skeletons tried to abduct the druid to avenge the death of the previous skeleton (Which had no relations to the gang of skeletons). When confronted by Capital City Guards (They are not pushovers, and carry magic spitting rifles that HURT) and the other Player Characters, the Lederhosen Gang went out in a blaze of stupid, wielding massive golden magic shamrocks, with strong knockback. When a gangster skelly died, their' big-ass shamrocks vaporized into lethal showers of molten gold, denying any looting of their ridiculous (and kinda nasty) weapons.

2:Dark Souls Mimic) It is literally almost the same thing as the 5e Mimic, but it has foldable and easily concealable (And Long Lanky) arms n' legs. They are a step up in regards to insanity. Simplified name is DS Mimic

2 examples) They were attacked by a Naruto Weeb DS Mimic, legendary variant (meaning they are stronger and unique), that made swashbucklers look slow. Died mid-leaf whirlwind to the breaking of its right kneecap. Its Deathquote, "Faaaaaaaaavaaa beannnnnnnnssssssssssssss blegh."

Osama-Bin-Mimic, legendary variant, Had a gunpowder aura, rocking an overclocked suicide bomber vest, lying in wait behind 42 consecutive back-to-back doors. Our barbarian opened the final door, got bitch-slapped, then hit it... Kaboom.

My point here is that as a DM, you must remember that you can take a normally weak monster, and turn it into the craziest thing imaginable. Remember Tucker's Kobolds? ;(

Bonus: There is a vengeful brotherhood of DS Mimics hiding in the deepest recess of the Elder Grotto (Ancient Deepwoods), 2000 strong, stockpiling volatile magic explosives of all and any kinds. Nobody knows about them, and they have yet to pinpoint where the random cryptic death-threats are coming from.