The question [Centaurs, do they have control of their bodily functions?][1] asks what I consider to be a legitimate question (albeit containing misconceptions about real-world zoology) about a fictional species that appears in RPGs.

Are questions about species appearing in roleplaying games that require research outside the rules in the specified RPG system on-topic?

EDIT:

To address Brian Ballsum-Stanton's comment regarding the meta question [Are campaign research questions on topic?][2], I believe that this sort of question *is* campaign research, as long as the system is specified, or other constraints specified.  In the example question "Centaurs, do they have control of their bodily functions?", the system *is* specified (D&D 3.5), and the aforementioned rules *do* provide some (albeit very scanty) information.  Certainly, the subject of the question may seem trivial and silly from the point of view of an outsider, but to the players, it may be an issue of great importance that could change the course of a whole campaign.  Who are we to judge? 

A question about real-world campaign research indeed may *not* be relevant here, though the consensus appears to suggest that it *is*.  And as this type of question is about fantasy species appearing in a specific RPGs, even if those rules don't say everything the players want to know, the rules provide a starting point that a person with other expertise can expand upon.  

Where but RPG SE is a role-player likely to find a person familiar enough with both the RPG in question *and* the other area of expertise?

  [1]: http://rpg.stackexchange.com/q/39545/8962
  [2]: http://meta.rpg.stackexchange.com/questions/570/are-campaign-research-questions-on-topic