Origin of "Jesus Christ it's a lion! Get in the car!"?

One thing I’ve found Google often fails at is providing me with the source material for a meme. Every Google result is inevitably someone USING the cliché; rarely is someone explaining the background of the cliché.

So, see subject line - where did this quote come from, that launched 1,000 lolcats?

Google this: “Jesus Christ it’s a lion! Get in the car!” etymology. It gives four hits, one of which gives credit to this

I’m so out of it, I’ve never heard of this.
A quick google search, though, turns up this:

Post #12 here:
http://forums.facepunchstudios.com/showthread.php?p=6493943