Evil Creeps are lane minions from the opposite team.
In the picture below you can see two different colored Minions. (Red and Blue)
In the blue are your allied lane minions, fighting for you and attacking enemies, turrets, and the kraken. They and the Kraken are the only things that will bring down the barrier on the turrets and the Vain crystal, they do this by proximity.
Red minions are enemy minions, attacking on the opponent's side. If you engage them, they may change their attacks from your allied lane minions, to you. They aren't allies, and will attack you, the enemy lane minions, and the kraken when it is unleashed on your team.