18.09: let's call it "Jackrabbit"

I was mainly considering Jellyfish and Jaguar (and Jackrabbit).
Originally I was inclined for Jellyfish, but then I thought of the
release T-shirts someone makes and it didn't seem suitable...
Jaguar would keep the name referring to a car as well, but as a
not-too-old (Mac) OS version is codenamed that way, I didn't go for it.