The DMG advises against allowing players to begin the game as monsters, and Demon Idol agrees. However, a player might end up with a monster character via magic such as reincarnation.

This page is a rules extension^Ex which provides rules for handling these cases.

Scores should be rolled randomly with whatever method (I-IV) player desires. Modifiers and mins/maxes should be customized by the DM to suit the form. The DM should come up with these for the form, preferably with the agreement of the first player to have a character of that form.

As an example, here are modifiers and min/max for a centaur form:

Charisma penalties are removed when dealing with their own race (just like dwarves and half-orcs).

These also need to be determined by the DM. Consider the monster, its relation to the setting, and what makes most sense. It can be good to give every monster at least one class in which they are unlimited in advancement. However, in the end, level caps depend on the setting, the game, whether UA caps are being used¹⁾, and what kind of game the DM prefers.

Use whichever is better between the character's class(es) and the monster's stat block.

For each category, use whichever is better between the character's class(es) and the monster's stat block.

Equal to the monster's AC or the AC of the worn armor, whichever is better. DEX mod is applied to either.

Monster characters who have at least one class acquire weapon proficiencies from their class(es) only; they do not automatically get weapon proficiencies for any weapons listed in the monster stat block. Conversely, if the character does not have a class, then the character has proficiency with any weapons listed in the monster's stat block.

Choose a race that best matches the monster's form. If no mention of longevity is made in the monster's description, just use the human age brackets (and starting ages, in the case of reincarnation).

In cases of reincarnation, the character should always have either common or telepathy, since otherwise the character would be annoying to play in a group. Outside of reincarnation, it's very possible that the magic which produced the new form did not grant the common tongue, or any standard tongue at all!

Most monsters don't have a listed carrying capacity, and the DM will need to decide.

In cases of monsters with listed carrying capacities²⁾, some extrapolation can be made.

As an example, in the case of a centaur reincarnation, the DM can assume the medium warhorse is the closest horse to a centaur, since their MV and attack damage matches up with that of a centaur. The DM could assume that a standard medium warhorse has, say, 16 strength, which would grant a human 700 cn carrying capacity without losing movement speed. Medium warhorses have a capacity of 4,000/6,500³⁾. 4,000/700 is 5.71, and since that is already a very large multiplier, it could be rounded down to 5. So a DM might say that a centaur character has 5x the carrying capacity of a regular character with the same STR score. As you can see, some measure of reasoning will need to be applied for such carrying capacities.

Human hearing can often be used. Elven hearing might be used for sylvan reincarnations.

All monster characters have infravision unless their stat block says otherwise. This is the same sort of infravision the regular PC races have, not the “monster infravision” with the 120' range and ability to differentiate objects. This is the case even if the monster normally has “monster infravision”.