Granted, Nintendo can have some creative liscence over what they do :/ Garchomp just looks like a land shark, I've never heard of a real-life land shark, and I doubt there is. Smuggy there is far more sleeker than Treecko, and Treecko just has some weirdo, gecko head (or one that ressembles an arse, possibly.)
I like the grass starter, and the picture did make me chuckle. I guess Torterra was a better Venasaur, and Meganium...a lanky Venasaur, so yeah, Treecko probably needed a skeeker form to compliment it. That, and the probably need to make sure that you can breed them together as well.
That, and if smuggy didn't have legs, it'd be a really slow starter, I suspect.