Reagain, along with Premantom has the highest base Defense stat of all non-legendary Dark-type Pokémon.
Reagain has several memes and emoticons written on its body. It has a penis (8=D), kitty face (:3), laughing face (XD), smiling face (:^) ), loss.jpg, and a smaller penis (.^.), a failed Triforce (Newfags Can't Triforce), as well as the word FUG.
Reagain may be a reference to Earthworm Jim.
Read the description again, it has something useful.
"Read again." Because you may want to read the description again. In adition, "re" and "again" mean roughly the same thing, so having them both next to each other in the same name fits the meaning they share.