At various times I've tried various ways to add a shadow and put their feet on the ground. But nothing's really worked, especially when they had no hooves.
The other day I tried this and quite like it:
It's very subtle but it's meant to be. Hopefully no-one will notice it, but it'll just indicate subconsciously that they girls are firmly planted on the grass.
Does it work? Does it highlight the fact that they only have two legs, and look a bit odd for that reason? Does it look odd that there's a shadow there but nowhere else?