These creatures have been mixed and matched with your online fun for a long time now. Here's a simple description of each kind of pet you might encounter.
Interactive Pets:
Pets that are attached to some kind of website using code that allow you to pretend feed, breed, play with, and level up (like an RPG character) via clicking and asking for other players to help. There are even sites now that provide the code ready-made for you like Squiby and My Adopts where all you do is supply the images and stages and the provider does the rest.
"Click-to-Feed" is where users use mouse clicks to level, feed, and play with a pet by choosing one of a few simple actions. It's a little like the old Tamagotchi method where you get simple commands and watch the numbers go up. Some people prefer this over more complex interaction. Examples of this method are Dragon Cave and Pokefarm, though recently even these sites add a little more add-ons and things to do during seasonal events and holidays.
Facebook applications have also used this for games like Pet Society where you create a creature, level it up, house it and purchase items for it using a virtual form of game currency.
Non-Interactive Pets:
The simplest, easiest to create and obtain form of pet is the "click-and-take". It's a premade image that's in the form of a basic image file (jpg, gif, or png) that you save to your own computer, reupload to a website and provide a link back to its original creator. Deviant Art seems to have adopted a form of this where collectors purchase pets either with real or virtual currency.
Some adoptable Groups on Deviant Art:
- ELLO-Adopts: http://ello-adopts.deviantart.com/
- Adopt Option: http://adopt-option.deviantart.com/
- Adoptie Shelter: http://adoptieshelter.deviantart.com/
Growing Pets:
Both interactive and non-interactive pets can have stages from egg/baby to child to teen/adolescent to adult. It's entirely optional. I didn't start making/coloring growing pets until I joined Gaia Online in 2003. My pets are usually single stage pets where even the offspring are single stage unless I'm planning on trading them for online/virtual currency via Gaia.
As an Art Form:
All this is entirely up to you as far as making, selling, and creating your own pets are involved. Don't let anyone tell you it has to be one way or the other. Mix it up and if you don't enjoy making them, chances are your pet adoptable will be short lived. If it's a simple distraction or something you plan to make a little real money on, be sure to let your adopters know exactly how you plan on using, distributing your pets right from the get-go to avoid a few conflicts later on.
You're always going to get that one person who's never satisfied and who will try to get you change your designs over and over just to please them. Draw the line somewhere or you'll never be able to move on to the next project. Lay down your rules as clearly as possible to avoid things like that and stick to them. Unless there's a big, obvious flaw in your system, you should be able to have a lot of fun and gain a little more exposure by making cyberpets.
