I've come across some examples of solo TTRPG enthusiasts attempting to create what they sometimes call a "Monster A.I.", which is designed to determine the behaviours of monsters when encountered in a game of solo D&D or the like. It got me thinking about OD&D and how in those earliest rulebooks monsters behaviour is already (albeit in a very simple way) codified to some extent.

OD&D Default Monster Behaviour for Solo Play

To begin with are some basics to setup a typical Wandering Monster encounter. Typically the circumstances of the encounter are determined as follows:

A roll for Surprise is made. To quote Underworld & Wilderness Adventures "A Condition of surprise can only exist when one or both parties are unaware of the presence of the other. Such things as ESP’ing, light, and noise will negate surprise. If the possibility for surprise exists roll a six-sided die for each party concerned. A roll of 1 or 2 indicates the party is surprised. Distance is then 10–30 feet." If there is no surprise then "Players will see monsters at 20–80 feet [...] So here we already have procedural generated some circumstances regarding the encounter.

The Underworld & Wilderness Adventures book then goes on to describe the behaviour of the monster(s). "If monsters gain surprise they will either close the distance between themselves and the character(s (unless they are intelligent and their prey is obviously too strong to attack) or attack.

We are also provided with a good example. "[...] a Wyvern surprises a party of four characters when they round a corner into a large open area. It attacks as it is within striking distance as indicated by the surprise distance determination which was a 2, indicating distance between them was but 10 feet. The referee rolls a pair of six-sided dice for the Wyvern and scores a 6, so it will not sting. It bites and hits. The Wyvern may attack once again before the adventurers strike back." You'll notice that dice are used to determine whether the Wyvern will bite or sting. This sets a precedent that if a monster has multiple attacks then a die roll can be used to determine, which attack is used.

The book also tells us that "Monsters will automatically attack and/or pursue any characters they “see,” with the exception of those monsters which are intelligent enough to avoid an obviously superior force." and "[...] the more intelligent monsters will act randomly according to the results of the score rolled on two (six-sided) dice." This is referring to the famous Reaction Rolls, which have been prevalent in all old school editions of D&D.

The book even tells us that "The direction of appearance is determined by random number generation considering the number of possible entries." So as you can see there is plenty of default behaviour cooked into the game that are ideal for solo play.

We are also given a useful guide for determining the exact number of monsters involved in the encounter. "if the level beneath the surface roughly corresponds with the level of the monster then the number of monsters will be based on a single creature, modified by type (that is, Orcs and the like will be in groups) and the number of adventurers in the party. A party of from 1–3 would draw the basic number of monsters, 4–6 would bring about twice as many, and so on."

The bit about deciding whether the monster is a single creature or the type that typically moves in groups is a bit ambiguous, however there is an easy way to determine this. If you take a look at the Monster Level Tables in Greyhawk (which was included as a correction to the original Monster Level Tables). The Monster Level 6 table is a good example, it includes the following entries:

Giants
Hell Hounds
Titans
Lords/Paladins
Hydra (9 & more heads)
Efreet
Dragons
Beholders
Golems
Evil High Priests
Basilisks
Wizards
Gorgons
Master Thieves
Fire-Breathing Hydra
Lich
Chimeras
Purple Worm
Vampires
Rust Monsters

You will note that some of these monsters are written as plurals and some singular. I believe this is done to tell the referee which monsters should be alone and which in a group.

So in summary we can say the following behaviours are the default:

If nobody (or both sides) gain surprise then monsters will automatically attack or chase (pursue) any characters they "see," unless the monsters are smart enough to avoid a clearly stronger opponent.

If the party is surprised then unintelligent monsters or those with a stronger force will close in on characters, or attack if they are already within 10 feet.  If intelligent monsters do not have the stronger force, they will act based on a roll from the "RANDOM ACTIONS BY MONSTERS TABLE" in D&D Vol. III.

All this being said there is still many gaps to fill if you want a workable procedure for handling monster behaviour in a solo game. However, it is quite easy to build on the information already given and there are also additional clues throughout the rules that might be utilised. Such as in the wilderness monsters who gain surprise will encircle the party. This could be adopted (if space allows) into the underworld, as is implied by the following text from regarding underworld encounter "There can be places where 300 Hobgoblins dwell, but how many can come abreast down a typical passage in the dungeons? Allow perhaps 3 in a ten foot wide passage, and the balance will either be behind the front rank or fanning out to come upon the enemy by other routes. The most fearsome man or monster can be overwhelmed by sheer numbers of smaller/weaker creatures provided the latter are able to close!"

We might also surmise that monsters can approach from any of the following random directions: either behind or ahead and depending on the monsters skills, it could be on the ground, ceiling or walls. There are also circumstances when it might approach from a flanking position if the space allows for it.

Here is a PDF of some suggested rules, which covers the direction of approach and number of monsters appearing.