You could define pests as those creatures of whom, sometimes, there are just too many. A few slugs and snails in your compost heap is good, the little blighters all over your garden is generally seen as bad. You'll never get rid of them all, but you can control their numbers.
Snail collecting . . .

I don't see cats as 'pests', but I know from visitors to this site that many people do have problems with cats in their gardens. It would be odd for me to complain about cats when I've chosen to be a cat-owner, but I can understand why their habits aren't always welcome.
Cats in your flowerbeds

Garden snail - photo by Eduardo Sabal.

Rosie the cat