1 Siobhan: Ishmael! I didn’t know you had two cats!
2 Ishmael: I don’t. Only one’s mine. Moby Tom.
3 Siobhan: What’s the other one, then?
4 Ishmael: He belongs to a student named Starbuck down the hall. That’s Moby Harry.

I've had this joke in my notes ever since Moby Tom was introduced back in strip #1659.

Uh. Yeah. I think almost 16 years is getting up there, but perhaps might not be the longest delay between an intended joke and the delivery in this comic.