You have to beat the game and then there is a small cave where Wiscash is and go in it. The you go up to the glowing thingy then press A then select Houndour by pressing A the Houndour with become a Houndoom. This will also work on any pokemon that you bring with from the friend place. Then you can evolve them. (Except for the pokemon that have already evolved, can't evolve, are too low of a level, or legendaries.) Yay! Now you know how to evolve not only Houndour but other pokemon as well!
O,O The Old, Wise Owl.
Gastly evolves into Haunter at Lv25.
When you trade him/her like in all other generations.
You'll need to trade for a Haunter in order for it to evolve into Gengar.
You don't need to get a gastly to evolve because you can catch haunters in Pokemon tower
You have to trade haunter to a friend to evolve it.
Haunter evolves into gengar once you trade him.
Gastly evolves into Haunter at Lv25.
you have to trade it back and forth with another trainer to evolve it
haunter does
Haunter can only be evolved into Gengar via Trading (such is the case in all Pokemon titles that include Haunter, not just Silver).
Haunter can be found on Route 8 or in the Safari Zone.
You use a haunter
Haunter is one of a few Pokemon that needs to be traded to evolve. A list of a few others Graveler, Machoke, Kadabra. Those are just off the top of my head. But you will need to trade it to a friend and have them trade it back.
Magmar,Electabuzz,Seadra,Poliwhril,Clampearl,Machoke,Rhydon,Slowpoke,Kadabra,Onix,Graveler,Dusclops,Haunter,
When you trade him/her like in all other generations.
You have to trade it to an other game for it to evovle.
Haunter doesn't evolve at any level to evolve your haunter you need to do a link trade with a friend and he he will then evolve