Depends on the world. Most are humanocentric for simplicity reason. For example, in Star Wars, humans clearly dominate (they are more frequent, and more spread-out, than all other species); just because it's easier to have human actors playing humans. It's also easier, for players, actors, writers, readers, etc. to empathize with humans, whose reactions and emotions we can guess and understand.
And it's easier to imagine humans -- or at least humanoids -- than totally alien creatures. Look at ET, or Alien, or Predator, or the Grey, or most other hollywood extraterrestrial: they have two legs, one torso, two arms, one head, and the face is usually human-like also, with a similar mouth, nose, eye and ear configuration.
Totally alien aliens are usually considered cheesy and unbelievable.
Even when this kind of simplicity is not needed (for example, in a RPG world), we have a bias of putting humans into a dominant position. Simply because we are humans, and we are proud and vanitous. Uncounsciously, we would not want to put us in a position of inferiority. Or, if we do, the rulers would be tyrannical and one of the objective of the story would be to overthrow the tyrants and free the human race. See Tshai, by Jack Vance, for an example of what I mean.
In my D&D campaign world, there is a dictatorial regime of "melnibonelves" (not really like melniboneans) ruling over humans and halflings. But humans also have two small independant countries; and the player may discover there that free humans are not necessarily any nicer... (Well, one player sumed-up my world by saying "it's the world where good guys are evil and bad guys are evil too -- save that one kobold tribe we found").
