Attack of Spongebob Squarepants?!?!?!?

Fellow TTDLXP enthusiasts,
 
I was playing some TTDLX today at lunch (still exploring --ALL-- of the
upgrades and enhancements).  I just happened to notice that SBSP was in
one of my cities.  I found it to be humorous especially as my son thinks
this game is for adults but he loves SBSP.  I took a screen shot of SBSP
and I have it posted here:
 
http://www.smd.cc/ttdlx/scr18.pcx
 
I am curious as to what triggers his appearance?
Are there any more of SBSP's friends (Patrick, Squidward, etc) in the
game?
Is this one of those rare events like seeing the submarine?
 
Please let me know!
 
Bill