To get Seviper, you have to have Pokemon Sapphire or Emerald (Zangoose is in Ruby) and you get it on Route 114 near Lanette's house.
No. Siviper is in Sapphire/Emerald
You must trade one over from Pokemon Sapphire or Emerald. It is spelled Seviper, by the way.
delibird, siviper, dialga, heatran, palkia,reshiram, zekrom, giratina,darkrai and much more.
it's hard to name all of them but here are the ones i know: torchic, treecko, mudkip are the starter pokemon: skitty, munchlax, tailow, wurmple, silcoon, castcoon, beautifly, dustox, (siviper i think) zigzagoon, and i know more but i can't name them all so ill give u one more and i dont know if i'm right but i think it's roselia.
The only way besides migrating is to talk to the girl in sandgem town after you get the national pokedex.Actually its not the only way. You can use cheat code devices, Trades, and you can just go to all the places were legendaries are found. Oh and also Events. - Not all games have sandgem town.