Plankton is his last name. His full name is Sheldon J. Plankton.
Xerxes
Xenophon
x-rays
Malcom X
In the episode, "Plankton's Army" Plankton announced his full name once he got to the secret formula. He said, "I, Sheldon Jay Plankton," So his middle name must be "Jay".However, if you look at the transcript for the epison, Plankton says, "Sheldon J. Plankton." Therefore, his middle name is not mention, but his middle initial is "J."
Well, there's a fashion house that starts with X if that will do--Xoxo
Floating marine organisms falls under the category of planktonic species. Hope this helps, x x x
Xavier
Xavier
Xavier
Xenophon , Xerxes , Xan , well xavier starts with an x. and im pretty sure its a human name.