It might sound like heresy to the old school crowd, but one thing I like about the 5e Monster Manual is how they give every enemy some sort of gimmick.
Read through the Labyrinth Lord/OSE/S&W/etc Monster Manuals and you'll get a lot of generic stat blocks of predictable HD/AC/ML and the sort of generic claw/claw/bite or weapon attacks you could have made up yourself. This results in the evergreen tip of
Just Use Bears.
Meanwhile 5e will at least give a generic Goblin the ability to run and hide as a bonus action.
Basically if you've run enough D&D and you're running an OSR module with not much prep time, give the 5e MM listing a quick glance to see if there's anything interesting the monster could be doing beyond 3 attacks at 1d6/1d6/1d8.
The other thing about running anything is that you've got about a hundred different things zooming through your head at a time, so if there's an ability it should be pretty easy to remember and simple to adjudicate, and if there's some sort of contextual trigger you should probably just tell the players as soon as seems reasonable (that way it's in their head instead) and/or tie it to something obvious like death throes or odd/even damage rolls.
You can also have a Recharge mechanic like 4e monsters - special abilities are recharged (and usually immediately used) on a d6 roll.
So in that vein here are some normal monsters.
Black Pudding
HD10 AC13 ML12
Menacing pile of jet black ooze that dissolves pretty much anything that's not stone.
Fears fire, but not light. Immune to acid, poison and cold.
When hit by a stabby or choppy weapon, splits into two smaller oozes with half the HP which each have the same attack.
Hits for 2d10 damage. Hits deal a
Notch to armour. Weapons that touch it are also
Notched.
Slow but can squeeze through very small gaps no worries. A terror.
Carrion Crawler
HD3 AC12 ML5
Squirming maggoty corpse eaters with a full 8 attacks from their 8 flailing stingers.
Bites for a mere 1d4, but that's cold comfort when you and your friends are paralyzed.
Deep Ones
HD2 AC14 ML9
4-eyed frog monsters.
2 claws for 1d6 each + paralyzing bite for 1d6: as much Pain Poison on failed Save vs Doom.
Long ranged hand-tongue attack - on hit they suck you towards them (for small things) or them to you (for brutal leap). Successful tongue attack means they get a free bite or wrestle at +4 to hit.
Deep One "Mermaid"
HD4 AC14 ML9
2 claw attacks for 1d6 + magic pheremones charm all who scent them.
She looks like the woman of your dreams. If you're not into women you get a +4 bonus to the Save vs Doom or be Charmed and do whatever she wants - usually get implanted with frogspawn.
1d4 hours gestation time (Roll 1d4 every hour - on a 4 it's time). Save vs Doom or rupture as the eggs erupt from your throat.
Eggs swell into Sea Salties after 24 hours immersed in water. A Sea Saltie that sinks to a great pressure will hatch into a male Deep One. A Sea Saltie that's swallowed replaces your tongue with a baby female Mermaid that will grow and slip out of your mouth into fresh water in a few weeks.
Swallowing a Sea Saltie gives you an hour of water breathing and pressure resistance from the unnatural mutagens. Further sea salties don't breed more mermaids though!
Dwimmerdragon
HD9 AC16 ML9
Wingless dragon-ish creatures made of Azoth-enriched stone, siblings to the Dwarves and originally designed as protectors of the Dwimmermount mines.
Alas, they cannot leave the Azoth area without falling ill and slowly turning back to stone.
They make Stone Sons of themselves, original masters of the technique.
They are chatty and vain, prone to flattery.
They are covered in pearls and gems, a side effect of the stone son procedure, worth 1d10*1000 obols.
Roll 1d6 for their attack each round:
1-2: 1d6/1d6/3d10 claw/claw/bite
3-4: Garlicy sulphur-breath deals their HP in a cloud and turns extremities of non-stone creatures to dirt.
5: Roll two spells and cast one of them as a L9 Wizard.
6: Sink into stone and regenerate 1d10 HP.
Gargoyle
HD4 AC14/20 ML7
Weeping Angel style menaces. While in line of sight they are statues - AC20, take 1 damage from non-magical.
When not seen they are blindingly fast dusky blue winged creatures that leap and strike with a claw/claw/horn for 1d6 each.
Smashing a gargoyle turns the area pitch black - as Darkness 15' Radius.
Gelatinous Cube
HD4 AC12 ML12
Practically invisible.
Pseudopod attack on all in melee range for 1d6.
Envelops stunned people for 1d8 damage/round. Wrestle at +8 to escape.
Giant Boring Beetle
HD4 AC14 ML6
Huge beetles with big mandibles. Bite 5d4. Drones on and on.
All Boring Beetles are linked in a giant hivemind. They act as one and all roll the same die - it applies to all attacking beetles.
If you are bitten by a Boring Beetle you are infected with their Boring Hive Mind and get the same roll as the beetles. This lasts until a Boring Beetle rolls under your Wisdom Score with an attack.
Giant Stag Beetle
HD5 AC16 ML6
Surprisingly stealthy. Giant stag beetle loves to grab and pinch and crush!
Two attacks. On hit, 1d8 damage and Wrestle at +10. Deals 1d20 damage in a Wrestle.
Can move around normally while Wrestling, hoping to drag prey into a dark corner.
Giant Tick
HD2 AC14 ML8
Huge arm-length ticks that crawl up you and stick their whole head in your body to feed on your blood.
Horribly fast.
Drain 1d6 HP and 1 Constitution from their victim per round. Drop off if burned, submerged, or killed.
Victims Save vs Doom after combat or contract the tick's paralysing disease - every day Save vs Doom or lose 1 point of Strength. Pass 2 days in a row to recover.
Green Slime
Drops on living things to turn them into more of it. Awareness check to avoid as it falls.
Deals 1d6 Constitution damage/rd (Save vs Doom for half) until scraped, burnt or frozen off.
Burning always works (and deals 1d6 damage to the recipient), the slime leaps away from the open flame.
Scraping deals 1d6 damage and the recipient gets a Save vs Doom to make the green slime get gone.
Destroyed by fire and sunlight.
Hell Hound
HD4 AC12 ML12
Stitched-together corpse frankensteins made from dogs.
Bite for 1d10.
Breathes flies and glowing green poisonous vomit in a cone for 1d8 damage + Notch armour and weapons. Recharges on a 1-2.
Can detect invisible with exceptional hearing and smell.
Horrible Unicorn Persons
HD6 AC15 ML8
Astonishingly fast (hence AC) and pale white. An attempt to make a perfect being that went horribly wrong. Won't attack virgins unless attacked first. Androgynous, nude, no genitals.
Double damage horn ram on charge, attack at +8 and wield great weapons that lend them +2 damage from strength. 2 attacks - 1 gore with horn for 1d8, 1 weapon attack usually a greataxe for 1d10+2. Punch for 1d6 in a pinch.
Phase Spider
HD5 AC14 ML8
Spiders whose webbing phases into the Ghost Dimension.
While on the webs, these spiders can phase in and out at will. Off the webs, it takes a Move.
Bite deals 1d6 damage and as much Pain Poison on a failed Save vs Doom. The venom phases you into the Ghost Dimension while it's in your system. Good luck!
Roper
HD10 AC16 ML8
Classic stalagmite monster.
50' reach with its 1d8 damage tentacles.
On hit, grabs you by the leg and pulls you through the air towards its maw. You've got two tries to escape the Wrestle, else 1d20 damage per chomp.
Rust Monster
HD5 AC12 ML6
Strange cockroach-like beasts with whipping antennae, rust monsters rust metal and eat the rust. Dreaded by all.
2 tentacle whip attacks deals 1d4 Notches to metal armour on hit.
Hitting it deals 1d4 Notches to metal weapons.
If it crumbles a weapon or armour to rust it'll spend a round eating it up, very cute.
Shambling Mound
HD10 AC18 ML12
A mass of moist vegetation around a supremely hard to hurt (thus AC 18) moist wet core.
Two clubbing limb attacks for 1d8 each. If both hit, auto-grapples and engulfs, suffocating you and dealing 2d8 damage per round.
Lightning attacks heal it for traditional reasons. Vegetation is too moist to burn so takes only 1 damage per die from fire attacks. 1 damage from Smashy or Stabby weapons.
Wight
HD4 AC12 ML12
Humanoid shapes full of shadow and burning black fire.
Immune to mundane.
Armour piercing 1d4 ghostly chilling touch + Save vs Doom or Level drain (down to just under level up). Inform the players of this as soon as they see the Wight and see how they react!
Anyone drained by a Wight becomes a 2HD Wight under its control, their soul dragged from their body by a spectral claw and turned to burning darkness.
Ravenous Zombie
HD3, AC14, ML12.
A zombie that ate a brain and became fast and intelligent. Glowing red eyes give them away.
Vicious and can speak, mostly to mock and coordinate.
2 claw attacks for 1d6 each.
Always burst down the person with the most Bleed dice, or otherwise least HP.
Ravenous Mode lasts 10 minutes, so usually walk around like a regular zombie until they spot prey.
Zombie
HD1, AC13, ML 12.
1d6 claw. Double HP. Slow as hell - they have a special third initiative behind everyone else.
If they eat an intelligent creature's brain - become a Ravenous Zombie.
Horde: +1 Backstab per zombie in melee range of their target.
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