Moshi Monsters is an online virtual pet community. Scratch can only make small online apps, and can't communicate with each other. You'll have to wait for Scratch 2.0 which has Cloud Variables. Ask me then.
Scratch can't play online with other players..... yet.... but you could still probably figure out how to make a small virtual world for the player to enjoy.
Kinda like this: http://scratch.mit.edu/projects/Beeman_98/2567296
