Shiny Kanto Suicune is possible because of HGSS. The OT/nickname thing is suspicious, though. I would suggest looking for it in pokecheck's database and check it's legality analysis. :)
EDIT: If you PM the OT, ID and Nickname I can check it for you.
Would a HG/SS say that it was caught in Kanto? I guess HG/SS confuse me having the two regions combined as one world.
If it was a Gen III from FR/LG it certainly COULD be RNGed(or lucky catch) to be shiny.... BUT there was a glitch in the roamers of FR/LG and the beasts will have HP be normal, but Attack can be no higher than 7 and the other four stats will be zero. (my lucky break on one of my play thrus was a beast that was perfect HP and 7 attack... pure luck I did not yet know RNG, and the roamers are WAY to hard to soft reset for).
I finally decided to officially take a break from breeding to work on my national Dex. I REALLY want to complete it before bank comes out. I decided to break it down this way: First I am doing each Kalos dex, then I will finish the "unobtainable list"(I am missing so few breeding familys, but have yet to evolve all of them... for example I have Meowth but not Persian). Then I will work on the ones that could come from Friend Safaris that are not in the Kalos dex(not gonna lie.... not a fan of raising happieness... Chansey is going to have a long wait to become a blissy). As I get bored of levling up I am taking breaks to GTS search for legendaries and missing ones from my dex. I lucked out and managed to get a shiny Dragonite earlier. I am finished the Central Dex, and only 15 away from my Coastal Dex (12 of those are ones I need to level up.... the other three are Eevolutions.... but Umberon and Espeon I want to just find on GTS rather than work on their friendship).