A little backstory seems to be required.
During the initial discussions when changing the name of the project from OpenBeOS (for fear of trademarks problems), someone suggested, jokingly, "we could call it Walter?".
Of course this was never meant to be serious. However, it resulted in the design of Walter the Fish, who later got his own website. For some time there even was a conference called WalterCon , and probably some T-Shirts were printed, too.
It isn't really the official mascot of Haiku, and this is now a forgotten story of the early days. But it has been sneakingly introduced in many places (some images got a "Waltermark" instead of a watermark, for example, or it can be used as an user name in some of our documentations).
It reminds people of the truth about Haiku: however nice it looks, no one really takes it seriously, it is an interesting technical experiment and adventure, not a production-ready system with a good marketing team behind it. Or at least, let the devs think it is this way, they don't work well under presure and prefer to do things for their own amusement.