The great thing about pre-2000 D&D is the simple monster creation. Any monster in the game can be adjusted on the fly with little work. Earlier this year I was running my AD&D 2E Campaign and half the monster didn't come from the Monstrous Manual. I was using Pathfinder Bestiaries and eyeballing their stats.
During my campaigns I like to add variation to a common theme. Fighting the same old orc, kobold, skeleton, zombie, etc gets repetitive after decades of gaming. Now taking a common monster and giving it a minor twist can add some mystery of the unknown to the table.
So Zombies are common low level foes. In OSR games they tend to have a couple of traits that define them. Let's use Labyrinth Lord as the base. They have 2HD, Immune to Sleep/Charm Spells, and they attack last in each round. For your basic zombie you can add tiny mechanical variation to help make your next horde stand out.
As with any additional special ability you should adjust experience as necessary.
Pick a variant or roll 1d20 for a splash of randomness.
20 Variant Zombies
1. Adhesive: The Zombies flesh is gooey and sticky. Any melee weapon that hits them get stuck to their skin. A character can make a strength check to pull it loose.
2. Embedded: These Zombies have glass shards embedded in their undead flesh. Any time a melee weapons hits the zombie they shatter. Each opponent within five feet of the zombie takes 1d4 damage from shattered glass.
3. Mold: This Zombie is infected with yellow mold. Any time it is hit in combat the mold releases spores in 10' cube.
4. Fused: Two Zombies are fused together. Double their HD and Attacks. (4HD, 2 Attacks)
5. Resilient: Zombies body requires a +1 or better magic weapon to hit. Its head is still vulnerable to non-magical weapons.
6. Groaning: These Zombie make loud groaning sounds. All within 30ft must save or be deafened. -2 to all rolls, spell casters have a 20% chance to miscast spells. Duration 1d4 rounds.
7. Vampiric: These Zombies drain life energy to help sustain them. Any hit will transfer any damage dealt back to them as temporary hit points. These hit points last for 24 hours.
8. Linked: A Horde of Zombies are linked to each other. Any damage dealt is evenly distributed among the horde. For example if you have 7 Zombies and one takes 6 points of damage you distribute it as 1, 1, 1, 1, 1, 1, 0. Zombies must be within 60ft of each other to be linked.
9. Exploding: Upon the final death blow this Zombie blows up into a hundred bloody pieces. All within 10ft take 2d4 damage.
10. Swarming: This Horde of Zombies swarm one target. The first opponent hit in combat is marked. All Zombie will attack that individual until it is dead. They ignore other adversaries regardless of their actions.
11. Infectious: Anyone killed by this Zombie rises as a new Zombie 1d4+1 rounds later.
12. Durable: This Zombie is extremely resilient. Max hit points per HD.
13. Imbued: These Zombies have had a spell fused with them. Once hit in combat it triggers the spell. For example if hit in combat a shocking grasp spell triggers. The following attack the Zombie is crackling with electricity.
1d4 Random Spells
3. Shocking Grasp
4. Stinking Cloud
14, Ice: These frigid undead deal extra damage and freeze your limbs. Deals an extra 1d4 points of cold damage. Also if it hits you must save or a random limb becomes frozen. The limb is useless for 1d4 rounds.
1d4 Random Limb
1. Right Arm
2. Left Arm
3. Right Leg
4. Left Leg
15. Dust: These dusty old undead cause respiratory problem. Any hit causes a cloud of dust in a 5ft area. It disperses after one round. A save is needed or you cough and sneeze for 1 round.
16, Possessing: The fight still lingers on for this Zombie. Upon its death blow its fading essence attempt to possess the nearest creature. Save or become a mindless undead for 1 round.
17. Mud: These muddy zombies slow your movement down. Each hit by this zombie encases you in mud. Each hit slows your movement by 1/4th. Once you've lost all your movement you are fully enveloped by mud and begin to suffocate.
18. Host: This Zombie has 1d6 giant maggots festering in its body. Each hit has a 50% chance of dislodging one from its body. The following round that maggot enters combat. Upon the Zombies death any remaining maggots burst from the body to attack.
19. Quick: With lightning speed these undead attack. Always goes first in combat, 2 attacks per round, and double movement rate.
20. Amalgam: Upon entering combat this Horde of Zombies converge on each other. They merge into one huge hideous undead abomination. Combine HD and Attacks for each zombie. 6 zombies would be 12 HD with 6 attacks.
Bonus21. Risen: Can't keep a good Zombie down. One round after being slain the Zombie rises once again. This time it only has 1HD.
22. Zombie King: This semi-intelligent undead leads the zombie horde. The more zombies part of the horde the greater its strength is. Start with a base zombie and add 1 HD per zombie. For each zombie slain you reduce the Kings HD by 1. A Zombie King with 8 Zombies in his horde would have 10HD. Also the Zombie King can transfer any damage dealt to another zombie within 30ft.