SpongeBob: Re-Hydrated (partially lost "SpongeBob SquarePants" animated short; 2003)
SpongeBob: Re-Hydrated (formerly thought to be called 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 Reloaded. 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 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 useful information there.
EDIT 4/29/2016: Found a clip of the short in Goopy Rossi's Vimeo account, along with a Jimmy Neutron promo.