http://www.youtube.com/watch?v=zuOIPo7OEoY#t=01m57s^Kaiserr steps up to the computer to modify a creature...
haha, oh man, that's the first thing I thought of. Sorry.
In all seriousness though, I never dreamed of trying to jump in and just create a whole new creature from scratch right out of the gate. I basically just modified existing things, making small tweaks, then making sure they actually worked and didn't crash the game or fail world gen, etc.
Then i'd go back and tweak more, and more, and more, and keep checking to make sure it was working. Whenever it stopped working, i'd go back and see what I changed to figure out why. Then i'd check the wiki for references to that specific token or whatever to see exactly how it is used and what it does, then do it right, and continue.
By the time I was done with something, it was nothing like the original, a completely new thing and you learn so much as you go and eventually you learn enough to actually create new things.
The monsters i've created at this point I couldn't tell you...
10 foot tall dog monsters with 5 heads and wings, with wool (for shearing), milkable, adamantine skin, acid blood, venomous bites, and so on. Perfect for guarding your fortress and supplying numerous useful products to dwarves at the same time.