Everytime I search arceus on the GTS, none is there. This is pretty obvious because WHO WOULD GIVE AWAY ARCEUS!!! But, if some nice guy (hacker?) puts an arceus on, it might have been traded so quickly that it goes before anyone sees it. So has anyone ever seen, posted, or watched a youtube video with an Arceus on the GTS?
Arceus is an event Pokemon. It cannot be put on the GTS, even if you want to. Other such event Pokemon include Mew, Deoxys, Shaymin, Manaphy, Victini, Keldeo, Meloetta and Genesect.
Hope I helped!