Allied monsters who players might want to bring on an adventure are usually handled by creating a new Race and/or Class… but why not just use the easier and more obvious solution: just play the monster from the monster section. Here's how it works...
A monster in B/X D&D is equivalent to a character of the same level as it has hit dice. A monster counts as 1 level higher for each asterisk it has for special abilities. So a Centaur (HD: 4) is equal to a 4th level character and a Vampire (HD: 7**) is equal to a 9th level character. This will help players and DMs decide what monsters can join parties of various levels.
Monster characters begin a 0xp and gain levels as a Fighter. However they don't gain extra hit points from Fighter levels until it's a higher level than their monster Hit Dice (and asterisks). Example: A Centaur would gain an extra d8 hit points at 5th level, while a Vampire would gain an extra d8 at 9th level.
The character uses it's Monster HD or Fighter level for to hit numbers and saving throws -- whichever is better.
They use either their monster AC, or calculate their AC for armour worn. A shield is always +1 AC.
They may make ONE attack a round using either a weapon or one of their monster attacks.
And that's it! They don't have attributes, or need special tables for their racial class, powers etc. Just use the monster entry. Now there's an easy way to play Centaurs, Ogres, Wood Golems, Living Crystal Statues, or Flumph!
