No, it’s not rare.
What happened to you is guaranteed to occur as a tutorial for the Dexnav in ORAS. Using the Dexnav, there is a chance a Pokemon will have an Eggmove; a move that usually can only be obtained through breeding. Using the Dexnav, you can hunt Pokemon with Eggmoves. The move selected on this specific tutorial Poochyena is based around which starter you picked, providing type coverage.
Hidden Pokémon can have their normal first move replaced with a special move, randomly selected from the Egg Moves of the first Pokémon in its evolutionary line. If a Pokémon has a special move, the DexNav will display an exclamation mark next to the First Move heading. A special move known this way can be taught at the Move Reminder if forgotten.
It turns out that this Pokémon, a Poochyena, is on level 5 and knows one of the Elemental Fang moves. The move it has depends on your first partner Pokémon; Thunder Fang for Treecko, Ice Fang for Torchic, and Fire Fang for Mudkip.
Source
ORAS Walkthrough
