Owlbear

The Owlbear is a joke until it drops the wizard

How to run it. The Owlbear is simple: 59 Hit Points, AC 13, and two Rends a turn at +7 for 2d8 + 5 Slashing each. No spells, no tricks. Its danger is that it goes straight for someone and hits hard, and with Speed 40 and Climb 40 it gets there.

How fast it kills. Take a level 3 wizard with AC 12 and 20 Hit Points. The Owlbear hits AC 12 80% of the time. Both Rends hit 64% of the time, and when they do, 97% of the pairs deal 20 or more. Counting the rare single Rend that rolls 20 or more too, the Owlbear drops that wizard in one turn about 64% of the time.

The choice that matters is the target. An animal with Intelligence 3 won't plan, but it will go for whoever is nearest, loudest, or bleeding. Pick one reason and stick to it, so players can read it and protect each other.

The common mistake is running it as a wall of Hit Points in the middle of the room. Start it hidden or climbing, so it arrives from above or behind. Darkvision 60 feet and Passive Perception 15 mean the party won't often surprise it.

For players. AC matters more than Hit Points here. Every point of AC takes 5% off each Rend. The fighter standing between it and the wizard is worth more than any spell.

The human angle. Owlbears make people laugh, and then they make people scared. McGraw and Warren's benign violation theory says we find something funny when it seems wrong and harmless at the same time. An owl's face on a bear is funny. It stops being harmless once the Rends land.

Source: McGraw, A. P., & Warren, C. (2010). Benign violations: Making immoral behavior funny. Psychological Science, 21(8), 1141–1149. doi:10.1177/0956797610376073