ok, so either Gyarados or Dunsparce or Murkrow
if those aren't the correct answer I officially give up just like for real life
edit: why these 3?
because it's obviously not Seviper, but something that has a connection with it
because that connection can either be being a snake-like specie or being a black-colored specie
because one of the hints for Seviper is Sevii
because Murkrow is the only black-colored specie spawning in Sevii (Pawniard is Red, Misdreavus is Grey)
because Gyarados and Dunsparce are the only snake-like species spawning in Sevii
edit 2: how in the world are we supposed to find Dratini with these clues, without just saying names randomly?