I think animals are just used as minor plot devices in Pokemon. They aren't going to show Pidgey eating a Wurmple of course, so they use worms, because everyone knows birds to eat worms and nobody really has a problem with it.
Besides prey, like in the fish example, they're just kind of there. Fish add to the theme that Cerulean's Gym is a Water-Type Gym, and back in the days of Kanto, they didn't have enough fish Pokemon to replace real fish with Finneons and Barboaches.
IMO, animals in the Pokemon world are considered "less interesting" or "less important", and thus are uninvolved in the action. I'd think that the only animals there are just the smaller, more prey-like ones, also. After all, there's plenty of references to fish, insects, and birds, but you don't see a bear or a komodo dragon or anything like that appearing any time.
Pokemon are just animals that can shoot lasers. We all know that animals eat each other. We have animals as pets, but we except that in the wild they eat each other to live. The thing about Pokemon is, they are so cute and have been given so many human like characteristics that it feels weird to think of them eating each other. They have a language and can even speak Japanese (English in america).
I personally thing that the Pokemon don't really have their own language and that Pokemon don't have the ability to speak ours. I always saw those things as something they put into the anime. Pokemon are animals that can shoot laser and nothing more, they eat each other to live. I have nothing more to say. I will take my leave.
I'm not too sure about the first one; animals are animals, and humans always have a taste for trying something new :x
Anyways, if there are other animals, wouldn't they be exterminated due to Pokemon being more able to fight?
Example being, SlowpokeTail from the GSC Generation; at least in the games. I kinda remember some guy saying it was pretty tasty, in the game.
But if you think about it, the Anime is a kid's show; they're not gonna make kids think that people slaughter Pokemon for food. However, if you read some of the 'Dex descriptions, Taillow and Starly are said to prey on Wurmple. So Pokemon eating Pokemon is somewhat used. But not in the show. Either the show adds them for effect of environment or to avoid kids seeing the true, gruelling nature of Pokemon, or there are normal animals in the world of Pokemon, which would explain all the meat references, and such.
Set by the wonderful Twinx
Platinum Glaceon Sprite is too cute! x_x <3