Wandering monsters in OD&D, from Dungeons & Dragons Vol 3: The Underworld & Wilderness Adventures by Gygax and Arneson, TSR, 1974:
Each movement turn in the underworld is 10 minutes, at the end of which the referee rolls 1d6. A wandering monster encounter occurs on a "6." (Note the use of "referee" throughout the original rules. The term "dungeon master" was introduced in the California gaming scene and made its way back to Lake Geneva by 1975.)
Monsters are divided into 6 "monster levels" for purposes of determining the encounter type. The referee first rolls 1d6 to determine which monster level table to consult, then rolls a different die to determine the exact monster type. It is possible to encounter a level 4 monster like a group of ogres on dungeon level 1, or a level 2 monster such as zombies on dungeon level 3.
Many of the encounters are with NPCs, described by the titles of different classes and character levels -- Thaumaturgists, Myrmidons, Superheros.
The "Thoul" on the level 2 table was not mentioned anywhere else in the original OD&D set. Moldvay's Basic Set (1981) seems to be the first core book in any version of D&D to describe this "magical combination of a ghoul, a hobgoblin, and a troll."
Among the level 4 monsters we find "White Apes," the 6-limbed creatures of Barsoom in Burroughs' John Carter stories that also could occur in the wilderness, and which still exist in 5e as the girallon. (Edit: Stats & descriptions of these also are missing from these rules, leaving the referee to adapt them from literature. Moldvay included white ape stats in his 1981 Basic rules, though he did not describe them having extra arms.)
The "Balrogs" on the level 6 table are one of several references to balrogs, hobbits, ringwraiths, etc that TSR mostly removed from reprints after receiving notice from the Tolkien estate, though some mentions in tables were overlooked and remained through all printings.
Further rules explain how to adjust monster numbers to suit the party level and size, when surprise occurs, how the party might avoid combat, and possible reactions of intelligent monsters to the party.