Question

Is Deoxys the first Pokemon over wi-fi event that is level 100?

Answer 1

No, there has been more level 100 Pokemon.
Black & White

Reshiram/ Zekrom
America: March 10th 2012 - April 8th 2012
Europe: March 10th 2012 - April 8th 2012

Deoxys
Japan: Not WiFi
America: May 8th 2013 - May 31st 2013
Europe: May 8th 2013 - May 31st 2013

Source: http://www.serebii.net/blackwhite/wifievents.shtml
http://www.serebii.net/black2white2/wifievents.shtml

Comments

Can you  name all the level 100 gifts, then?

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There weren't more, only these two