You can't.
I have no Idea about it on Ruby But I do know its on emerald. But My brother was talking about Deoxys on Ruby so Its a 50/50
you cant get deoxys in platinum dumbnuts only emerald ruby sapphire so be smarter dumbnut!!!!!!!!!!!!!!!!('_')
In Pokemon Ruby there is the 3 regis, rayquaza, latios, groudon, deoxys, and jirachi. So there are 8 legendarys.
I don't think you can but if you have Deoxsys on Pokemon fire red leaf green or ruby/sapphire emerald then you can transfer it to your Diamond
deoxys didn't exist until generation three (ruby, sapphire, emerald) so is impossible to get in crystal, which is generation two :)
with a pokeball
I found a Pokemon website called Serebii.net, and I also found out that in Ruby and Sapphire. You have to trade from Fire Red, Leaf Green, or Emerald to get Deoxys. So people who post questions about catching Deoxys in Ruby and Sapphire are the real liars. For those of you who posted answers like that. STOP MAKING LIES YOU TWITS!!!:@ :@ :@
you can ethier catch them or use acheat when you get so far in the game. you catch them. they are kyogre,deoxys,castform,regirock,registeel,and regiice.
Leafeon is a fourth generation pokemon, so you cannot own it in ruby (a third generation game), let alone catch it.
I don't really think you can. The thing is, in Sapphire you can catch Lotad but not Seedot and in Ruby you can catch Seedot but not Lotad. However in Emerald, you can catch both. So You'd have to trade from a Sapphire or Emerald. Lotad can be found on route 114 on Pokemon RUBY!
You would have to start a new game and catch the Pokemon again.
So far, the only way to get Deoxys in Pokemon Platnum (or in any Pokemon game for that matter) has been through Nintendo events.