This is a fun matchup, but Raikou would likely win in a fight against Onix.
Speed advantage: Raikou is incredibly fast and agile, while Onix is slow and bulky.
Type advantage: Raikou is an Electric-type, which has an advantage over Rock-type Onix. Electric attacks deal double damage.
Powerful moves: Raikou can learn Thunder, Thunderbolt, and Wild Charge, which would devastate Onix. Onix has strong defense but fewer offensive options.
Weaknesses: Onix is weak to Water, Fighting, Ground, Steel, and Grass attacks.
Onix's high defense could make it tough, but Raikou's speed, type advantage, and strong moves make it the likely victor. An upset is always possible depending on moves, strategy, and luck.