By popular demand, more vanilla D&D monsters get the scary treatment.
Ogres join the ranks of "things that want to eat you", but are much more sophisticated. They have elaborate methods for slaughtering their prey, often leaving them alive for the first few courses. They refuse to waste a scrap of meat or bone and practically everything they own is made from a part of some unfortunate victim.
Hobgoblins are invaders from a distant continent. Next to their colossal empire the known world is a minor continent. Their technology is always slightly ahead of humans. When we had bows, they had crossbows. When we had muskets they had Gatling guns. Their ships are ironclad and their military strategies incredibly cunning.
Owlbears are the Jason Voorhees of the animal kingdom. They stalk the woods at night, killing anyone that disturbs their peace. They're far tougher than they seem, but will play dead or flee when a fight is going against them. If this happens they will obsessively track their opponents and attack when their quarry is weakest. With extremely acute scent and vision it's near impossible to lose an Owlbear that's following you.
Gelatinous Cubes are already terrifying in life. In death their structure collapses to flood their surroundings with a mix of paralysing toxin and burning acid. Anyone that succumbs to both of these fluids will lie very still while they slowly melt.