cause when I battled it, it was shiny! is that supposed to happen?
No! Congrats you have a shiny that isnt Red Gyarados!
Hope I helped. :)
Source: I would have remembered very clearly if mine were shiny.
It is possible for it to be a shiny but its not found as a shiny like the shiny Haxorus in BW2 say, however there is a 1/4096 chance that it would be !