For example yesterday my level 7 group, which was already weakened after the had to retreat fought against two naunets.
First the naunets used dimension door to get to the fleeing party. Then they used their solid fog to impede the parties retreat which allowed them to down the parties rogue and cleric. When the parties sorcerer tried to escape, one of them used his tail attack to grapple him. In the end he managed to free himself and lure the naunets away so the barbarian could pick up the rogue and the cleric. But it was a very close call and the naunets definitvely managed to show what they’ve got.
I ran a customized monster in 5e. A siren. It’s ability was a straight copy from the Banshee but instead of knocking unconscious, it caused the player to spend their full speed running towards the creature.
The party was at sea and heard them coming.
They ended up lashing themselves and the crew to the masts and railings, giving disadvantage on a lot of stuff.
They were prepared and they won, but damn it was cool to see the party do exactly what they were supposed to do against sirens.