Happy's Little Job 3

Happy's Little Job 3 is an omake of Fairy Tail by Hiro Mashima.

Happy and Lucy go on a job that they have to exterminate (poisonous) rats in an old house.

Summary
Mirajane indicates to Happy a job that involves getting rid of rats that infested an old house, since he's a cat. However, the rats breath a poisonous gas, then Lucy goes with him to complete the job.

The duo comes to a big, abandoned and deteriorated house, where they should hunt the rats. Wielding a flyswatter, Happy becomes determinated to complete the job, but reviewing the scenario, Lucy wonders if it wasn't better to have Natsu with them. Inside the house, the two encounter a myriad of small fatty rats. Instead of being hunted, the rats start to chase Lucy and Happy until the last floor of the house, where Lucy decides to finally summon a Celestial Spirit to help them. However, she then realizes that she forgot the keys, much to their despair. They hide in a room to decide what to do, but the rats start to expell the poisonous gas inside the room, which makes the duo even more desperate. Happy then decides to back to the Guild to get Lucy's keys, leaving her to slowly faint alone by the gas.

Trivia

 * Despite not having an anime equivalent, a scene of this Omake is shown in the second opening of the anime, the only difference being the presence of Erza, Natsu and Gray inside the house.