So reviewing the centaur race from races of the wild I note that the race has a +2 level adjustment. It also states that thy start with 4 monstrous humanoid levels granting 4d8 hd. Does this mean a centaur begins play as an effective level 6 character?
Here is the rule for effective character levels, from the SRD:
To determine the effective character level (ECL) of a monster character, add its level adjustment to its racial Hit Dice and character class levels.
Your centaur is effectively a 6th level character before taking any class levels. If they do take a class, they treat their "6 levels" of centaur as a favored class.