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Someone with no brain or common sense, they usually travel in packs of three to accompany each other with self esteem issues commonly present in the derpii,

A single Derpy(derpii for plural) can be lethal in full rage, ways to prevent the derpii from entering this state

1, give it beef jerky, beef jerky is a natural relaxant of the derpii and will instantly calm it into a suppressed state.

2, another key way to stop a derpii from going ballistic is to impress it with a neat flip ( or dive) off a diving board, it will distract it for a short some time as it watches the leap from the board, at this point it is easiest to sneakup on the derpy from behind as so to knock it out with a quick blow to neck.

the following actions are how to enrage a derpii,

1, as it is swimming ( which it can just so hardly do ) run over to it and yell "derp!!!", it will quickly become enraged and most likey to attack you or the people surrounding you.

2, to be seen walking or near a Mariah will quickly enrage the derpii as it will become instantaniously jealous and attempt gainers off the diving board to impress the Mariah.
Jean: "Hey look Mat, theres another Derpii!"

Mat: "Wow thats the 5th this month!"
by Lily Pearson January 27, 2010