Are there any generic rules in D&D 5th for playing monsters as characters?

Thought some up myself and wanted to compare what others have come up with.

There are the sidekick rules in Tasha’s, but they live up to their name in that they are just inferior to a normal character, meant to literally Help, cast buff/debuff spells, or tank. You could use the sidekick rules with a normal player class if the dm wanted that kind of thing.

 Thanks for noting that.

But I was pondering more about some rules that one could start as a level 1 ogre, level 1 displacer beast, or level 1 mind flayer or level 1 beholder or any other monster, preferably so that it is simple to do and only unbalanced and not unbalanced totally out of proportion. And that with further level gain the full power of the respective monster can be realized (with potentially adding some class levels in between or afterwards).

(I have that idea from Pathfinder, where in some obscure rule it was stated, that you can be a monster of CR X, by starting at character level X and spending that X levels to be that monster; but that does not help with starting as level 1 mind flayer and does not work well as CR does not transfer 1 to 1 in power into character level; a lev 7 character is one on one no match for mind flayer)

edit:

for 3.5 there is savage species:

https://forgottenrealms.fandom.com/wiki/Savage_Species

but that is an entire book.

Yeah there is no way to (within the existing rules) to as a player scale down/up a monster for each level.

A dm might do that, if playing as a specific monster is the point of the campaign or something.

Basically, there is nothing in the rules to support it, and you absolutely shouldn’t expect it as a player, but as a DM, if you are willing to put in even more work and accept that balance might be a little wonky, you could set something up.

I just finalized some sort of rules.

Applying them to monster manual monsters

Aarakocra (i know there is a player race option for that)

Bugbear

Centaur

Barlgura (Demon)

Beholder

it works out that their lev 1 versions fare rather equally in 1 on 1 vs a single goblin, though all probably a bit better than a lev 1 fighter.

Basic idea is to give monster some amount of total levels based on its CR (by cross checking which character level is necessary so that the CR XP does not count as deadly) and for each monster level lacking to total level, penalties for char and bonus for opponents acting against char are applied.

For example the beholder level 1 rolls every non-damage roll at -9 and all damage rolls at -27 (minimum 1; meaning damaging eye rays often deal just 1 damage), while the goblin gets on rolls vs lev 1 Beholder +8 (meaning the eye rays are only effective DC 8) and + 24 on damage rolls vs beholder level 1 (meaning just 7 hits from the goblin kill the beholder and goblin has 80% hit chance vs beholder level 1)

(yes, i know, somewhat crazy)

Volo's guide has some rules about this topic. It is mostly tied to the races provided into that book but it can be used a base.

Never mind, took another look since it has been awhile since I've opened Volo's and it mostly referred to the RP side of playing a "monstrous adventures"