You found a Legendary!
A rare Pokémon has appeared on the map. Don't let it get away!
Walk with the arrows around your character, the arrow keys or WASD — Q/E/Z/C and the numpad step diagonally. Enter or Space talks to whoever is beside you; the apostrophe key (') toggles fullscreen. Trainers stand their ground — beat them to get past. On phones, drag a thumb across the map to walk that way. Openings in a map's border lead to the neighbouring map. Some Pokémon hold items when caught, and your steps can rarely turn up loose items too — they go straight to your bag. Carrying a Repel? Press R or the Repel button to use one and walk without wild encounters for a while.