SpongeBob: Re-Hydrated (partially lost "SpongeBob SquarePants" animated short; 2003)
The Spongetrix is a lost SpongeBob short that aired on Nicktoons in 2003. So far, it is known to be apparently a parody of The Matrix. According to an anon on a /co/ thread who claims to have the short on a VHS tape, the short lasted 10 minutes and came before an episode of Jimmy Neutron. Only two screenshots have surfaced onto the internet from a Nicktoons promo bumper in 2003, though it's not known when in that year.
EDIT: The original /co/ thread was deleted, but since the reference link is from Desustorage, an archival site for /co/ and threads from some other 4chan boards, it can still be viewed from the link that's posted.
EDIT 3/19/16: There's a new thread about this short on the Spongebuddy Mania forums that was created three days ago, but there hasn't been any replies to it so far. Hopefully we might get some more information there.7
EDIT 4/29/2016: Found clip of the short in Goopy Rossi's Vimeo account, along with a Jimmy Neutron promo.