Re: Seem legit?
Doesn't look like you can catch Magby or Magmar at Route 1 at all, so it looks like it's hacked, though I messed around with an IV checker and the stats look possible.
You can sometimes tell by having a Wi-Fi battle (with a friend or a random match up) and then trying to save and upload a battle video. But then again, if it got through the GTS, it might not be able to be picked up by Nintendo.
I don't think a Pokemon traded through the GTS could mess up your game. It does go through the official servers and everything. I don't usually trust anything I get from the GTS either, but people do get good deals sometimes so it's hard to know.
In this case the Route 1 thing is a dead giveaway. =)