You go to the namerater's house located in slateport, show him the Pokemon, enter the new nickname.
Note: The nickname must be 10 letters or less, and you can't rename it if it was received in a trade, either ingame or with a real person.
The Pokemon Name Rater is in Slateport City.
Slateport city on the northeast side. He'll rate your pokemon's name and change it for you if you want.
in slateport Dummy
Slateport city.
yeah no change with that
Pokemon Ruby has Pokemon that Pokemon Sapphire does not and Pokemon Sapphire has Pokemon that Pokemon Ruby does not.
In Slateport City, to the left of the Pokemon center.
its in slateport below the Pokemon contest buliding
You can't change it's form in the third gen games, it changes depending on the game - sapphire is normal form.
There is no Pokemon mansion in sapphire.
Slateport in front of the contest place
Ash