Deoxys is changed into speed form by trading it to Pokemon emerald or by walking up to a meteorite in veilstone city in Pokemon diamond/pearl and pressing A while it is in your party (first slot).
You can't get all of the forms of deoxs in Pokemon emerald that's what I heard from my freind.You can only get the speed form of deoxs.If this isn't true then I'm very sorry.
No, you cannot. Only if you have an action replay, and have the code to get deoxs. If you get the code go into the grass (in the game) while holding select and you get deoxs.
In which game?
Get the aurora ticket.
Deoxys is Phychic type. It has four forms: Normal, Speed, Attack and Defence, which can all be changed by having it in your party and touching the meteorites in Veilstone City.
Running
action replay or transfer machine
In Meteor cave, you can get there after recruiting Lugia
when you decrease or increase speed you are losing or gaining kinetic energy respectively
Go to Veilstone City and interact with 1 of the meteorites there, that'll change Deoxys' form into Attack, Defense, Speed or Normal Forme.
In veilstone city at the east there will be four meteorits that each have the ability to change the form of deoxys that are: (attack form), (defense form), (speed form), (normal form).
To Change Deoxys's Form You Need To Go to The Southwest Part Of Route 3,with Deoxys In Your Party,Stand Near The Top-Left Meteorite To Change Its Form To Normal,Top-Right For Attack Form,Bottom-Left is Defense Form,And Bottom-Right For Speed Form.