by the 6-year-old DM1. Give Monsters a Reason to Fight - Why do they care enough to enter battle? Do they want to eat you? Are they protecting their lair? Do they have babies? Are they just mean and evil? A monster that has a reason to fight is more likely to fight with passion and make things more interesting. They might also fight to the death!
2. Give Monsters a Personality - They are more than just hit points and attacks. They should be unique. Make them weird - give them flaws. Maybe they always make a bite attack when taunted. Maybe they hate the color red. Maybe they love the smell of spell components or guts. Describe what your monsters do so the players can see they are different than every other monter they have fought.
3. Make Monsters Scary - Maybe the party has been being hunted by the beasts. Maybe they have smart attacks and fight with strategy. Maybe they use magic or magical effects like a stinking cloud or death rays. Maybe they haunt dreams or attack at night or drink brain juice. Maybe the characters find a bunch of corpses that the monster already killed or maybe they have some people captured.
4. Make Monsters New - Monsters that the party already knows can be fun, but they are not as interesting. Make new monsters that no one has any idea what they do. Give old monsters new abilities like magic or feats. Blend two monsters like an owl bear with roper tentacles. Give your monsters unique tactics that surprise your players.
5. Give Monsters a Heart - What would your players do if the owlbear surrendered to them? What if the displacer beasts decided to take it easy on the party? Maybe the monsters respect the abilities of the party of even want to join them. Maybe the monsters decide it isn't worth killing the party and they take them prisoner instead… or simply leave them after the threat is over. Maybe the monsters are just hungry and will make great pets once they are fed.