PJ is a creature of unknown origin that seems to live in badfic Author's Notes. He/she/it is one of the two creatures most often mentioned in badfic disclaimers, as in; "LOTR belongs to PJ tho i wants it, prcious!!1!!111! Leggy iz hott!!1!" It is possible that the author is referring to Peter Jackson, critically acclaimed director of the Lord of the Rings film trilogy. This possibility is still under investigation.
An alternative theory is that badfic authors are referring to the character PJ from the popular 1990's children's soap Byker Grove, though what Ant McPartlin has to do with Lord of the Rings is anyone's guess.
PJ or PJs is also a shortened form of "pyjamas", though it is doubtful any work of fiction belongs to a piece of bed wear.
Such confusion about the identity of PJ could potentially have the effect of making the canon spawn something that resembles an unholy cross between Peter Jackson and Ant McPartlin, wearing pyjamas, although this has not been documented. Yet.