Long ago, I picked up a copy of The Dungeon #10 at my hobby shop just as they arrived. This was the October 1977 issue and was one of the first issues of The Dragon magazine to be available at Dibbles. This issue had a couple of good articles and the Snit Smashing boardgame -- the first Tom Wham game published in The Dragon. The best article in the magazine from my point-of-view was "Random Monsters" by Paul Montgomery Crabaugh. Two pages full of tables for rolling up random monsters. Not random monsters as in what standard monster appears on Dungeon Level 6 on a roll of 8 or what monster appears in a forest hex on a roll of 12, but a method of creating completely new monsters that had never been seen before.
I rolled up a lot of them and wrote each one on a 3x5 index card. When I needed a new monster for an encounter, I'd let a player draw a card from the deck and that creature was what they ran into. As the discovering group, they would get the honor of naming the monster if they defeated it (or at least managed to survive. Most of these random monsters were seldom, if ever, seen again. A few, however, became regular "standard" monsters in my campaign. I've listed of few of those below. The descriptions as rolled up from the random monster card (yes, I still have most of these cards). The names were all given by my First Campaign Arn group. You should be able to take this "card descriptions" and write them up for whatever system you are using. Consider these Open Game Content.
Highly Intelligent, Neutral, Reptile
Speed: 9", AC: 3, HD: 6d8+1
Special Features: 2
-- Breathes Lightning: 2d6 damage, 2x/day
-- Sting in tail, does paralysis (save okay)
Looks like a giant scaled toad (dark with occasional yellow patch of scales) with a scorpion like tail with a glowing stinger. About 12 feet long, 50% of that is tail).
Unintelligent, Chaos, Undead
Speed: 6", AC: 7 HD: 2d8
Turned as a Zombie, Destroyed by holy water.
Red-skinned humanoid with rotting flesh. Mindlessly obeys last order given by their creator. Their touch causes 1d6 painful rotting damage, turning the area hit to an ugly red until healed or washed with holy water (which also eliminates the pain).
Unintelligent, Lawful, Mammal
Speed: 6" (Fly: 18) AC: 8, HD: 3d8
Damage/touch: 2d6 fire
Special Features: 1
-- Fiery Aura surrounds creature (think Human Touch), damages all it comes it contact with
(This was rolled as "does double damage" and was my explanation for why)
Small bat-like creature with a 3 foot wingspan. At night, it looks like a flying fireball from a distance.