Most people know that Windows 95 was code-named Chicago. The subprojects of Windows 95 also had their code names, in part because code names are cool, and in part because these projects were already under way by the time somebody decided to combine them into one giant project.
|16-bit DOS kernel||Jaguar|
|32-bit DOS kernel||Cougar|
Even when they were separate projects, the first three teams worked closely together, so the names followed a pattern of ferocious cats. My guess is that when the user interface team chose their code name, they heard that the other guys were naming themselves after cats, so they picked a cat, too.
I don't know whether they did that on purpose or by accident, but the cat they picked was not ferocious at all. Instead, they picked a cartoon cat.
Bonus trivia: When the feature to show a special message after Windows had shut down was first added, the shutdown bitmap was a screen shot of Ren and Stimpy saying good-bye. Fortunately, we remembered to replace them before shipping.
If you were paying attention: You would have noticed that code names get reused a lot, not because of any connection between the projects but purely by coincidence.