Earth ponies are slightly stronger than the rest and have a natural talent for agriculture.
Pegasi can fly and walk on clouds as well as manipulate the weather by physically moving clouds.
Unicorns are magical - a typical civillian will have light and mage hand at will, plus one or two spells corresponding to their special talent. A competent unicorn will have telekinesis at will instead of mage hand and dedicated wizards can have much more.
However, most mentally enslaved ponies will lack creativity to use their special abilities. If you want more monsters, I think you still haven't used the Sass Squash I linked a page or two ago.
I have also found another monster - cloud gremlin. These guys can manipulate clouds like pegasi do and use it to mess up the weather to make everypony miserable so they can feed on their negative emotions.
