It was either this or a shark, but I think someone's already done the shark, so here's mine.
It's got one servomotor to tilt the ray left and right, and two motors that power the ray's fins up and down as it "swims".
UPDATE: Here's the video of the manta ray in action. Nothing fancy, just a swimming motion.
Edited by Reventist, 06 April 2012 - 09:52 AM.