I really can’t tell if you’re asking about how rare a Raikou you already have is, or how hard it would be to get one, so I’ll answer for both.
If you got a Raikou in a trade, and are asking how rare it is, then if it’s level 30 (as specified by a previous edit of this question), then it must come from a Gen 4 event back in 2011. Every game that has Raikou available normally has it at a minimum of level 40. If it came from this event, that would normally make it pretty rare, since it is quite an old event, but a DS exploit has made it possible to receive this event again from a 3rd party server an unlimited number of times. Even if it came from the original event, the exploit has made it less rare and diminished its value.
If you’re talking about catching a shiny Raikou from Ultra Space, then it’s no different from catching a regular Raikou, except that you have to soft reset for it to be shiny first. It’ll be full odds (or charm odds, if you have the Shiny Charm), so that’s 1/4096 (or 1/1365 for charm odds). Also, keep in mind that Raikou is exclusive to Ultra Sun.