Isaac the Tiger & Willasaurus Rex:

It started about a month ago when will Started growling while wearing his dinosaur pjs.

The game evolved (as did the depth of Will's roar) to:

Mom or Dad: Oh No! It's a Willasaurus!
Will: Roar!
Mom Or Dad: Ahhh! {with frantic movement} SO SCARY!!!

Now Will Roars the moment he hears: "Oh NO, Oh, Scary, Scare, Saurus, etc..."

Isaac decided to get in on the game a couple weeks ago.

His roar isn't as advanced as Will's but it's coming along. We call him Isaac the Tiger.

Tony Petty captured this well and put it on his photoblog (see below)

Willasaurus REX

and here's a good picture of Isaac
(although not roaring, he is wearing stripes)