Surely Groudon is far too busy arguing with Kyogre to have another rival–

[And that's where he stops, because 1) the Groudon/Kyogre rivalry brings back stressful memories, and 2) legendary Pokémon are certainly powerful enough to multitask. Even when it concerns rivalries.]

At least we'll know quickly enough, if territorial disputes are a risk. I doubt they'd be subtle about it.