The heads on their arms do not have brains. They use all three heads to consume and destroy everything.
– Pokemon White Pokedex
Hydreigon is the pseudo-legendary Pokemon of Gen V. It’s the only pseudo-legendary that has higher Sp Attack than Attack. Hydreigon evolves at the highest level of any Pokemon that evolves by level, at level 64. It has 3 heads, which is not uncommon in the Pokemon world (Dugtrio, Dodrio, Magneton etc), but according to the Pokedex entry, the two smaller heads are more on the “arms”, and don’t have brains (or teeth), so those heads are more like hands than heads. Those heads remind me of sock puppets.
Even though it has wings, it’s not part flying, but it does have Levitate as its ability. Because it’s also Dark type, this means that it has two immunities, to Psychic and Ground. It also means that it’s not doubly weak to Ice, like many of the other Dragon types. Though I find its Dark typing makes it easier to fight against, because it’s weak against Fighting and Bug (it dies quickly to Bug Buzz from Volcarona).
In the game, you don’t encounter Hydreigon until you fight Ghetsis. His Hydreigon was annoying, I remember I had some trouble with it. My Zekrom wasn’t fast enough, and got killed with Dragon Pulse. I sent out my Darmanitan because it has Brick Break, but his Hydreigon used Surf and killed my Darmanitan. I managed to defeat it with Archeops’ U-turn (but it took 2 turns, so I had to sacrifice another Pokemon).
Stat-wise, Hydreigon has 600 base stat total. Its best stat is in Sp Attack, but its Attack is pretty good as well. It has good Speed, but relatively low Defense. It has a decent moveset, though lacking in the ability to learn a Dark type special move (there aren’t that many to begin with). It can Dark Pulse by breeding with Arbok, Ekans or Seviper, but since Dark Pulse is no longer a TM in Gen V, the Arbok, Ekans or Seviper would have to learn it via TM in Gen IV and brought over to Gen V.