If you can, take it to the nearest pokémon center. That may be odd to say and difficult to follow, but you must keep in mind that you’re not a medical professional, whereas the staff of a pokémon center is and will know how to treat serious injuries. Furthermore, pokémon nurses are trained to not ask too many questions about a pokémon—especially one that may not be identifiable—so you can trust your local Nurse Joy to help. If you can’t bring Raikou to the center (and do not attempt to catch it in order to do so, as this can be an extremely bad thing), call the center and arrange for a home visit.
Best of luck, anonymous! We’re counting on you.