It is a Zoroark in disguise. It can be caught but must be done through events.
In order to catch Zoroark, the player will need one of the Shiny legendary beasts to activate a special in-game event. Approaching the trailer in Lostlorn Forest with one of the Shiny legendary beasts in the party will initiate the event, causing the mysterious, silent woman to emerge from the trailer with a screeching growl and attack the player.
When the battle starts, the player will suddenly face one of the normal-colored legendary beasts that has a type advantage against the Shiny legendary beast in the player's party (Raikou will face Entei). Once the non-Shiny legendary beast receives damage, the illusion will dissipate, revealing Zoroark.
After the player catches or defeats Zoroark, the true expanse of Lostlorn Forest is revealed, and the same Backpacker reappears and becomes surprised that the suspicious woman was really a Zoroark in disguise, who was only protecting her home by deceiving both people and Pokémon using illusions. He then states that Zoroark reacted strongly to the Shiny legendary beast in the player's party, and assumed that she may have been in some sort of a previous conflict with it.