From Volume 2 - Chapter 13 / Season 1 Episode 6. In a swift action, Luffy defeats Mohji and his fierce beast Ritchie. But it's too late. Chouchou has already lost his treasure. A somber conclusion, it is. But despite his small size, this dog has the pride of a warrior!
This arc is pretty forgettable, aside from introducing Buggy and Nami, but it does establish that Luffy respects other's dreams and ambitions, even those of an animal, as long as they're willing to put their life on the line. Even though Luffy is fundamentally selfish, he's also deeply compassionate.
Does this arc MATTER in the long run? No, not really. But it does let Oda build a rich world, by giving Boodle and Chou-Chou a whole backstory, and also allows him to show Luffy's character rather than tell. Not every author gets the incredibly long runway Oda has had to gradually shape his characters, but he's really made the most of it.