Origins

[Rappunzill]

This is going to drive me nuts.

Does anyone know the origin of the phrase, "That's the bunny!"
Is it unique to Terry Pratchett's Discworld, or is it some older/obscure British phrasing of which I am unaware?

[Rapscallion]

I tend to use "That's the *random animal*", or even *random object*. It's funnier if you can come up with something truly absurd.

Rapscallion

[ArcticChicken]

I think I started using it before I started reading Pratchett, but I can't remember.