Hmmm
Water soluble is a good start so cheap poster paints may do.
Be warned though - I have drawn on a horse with normal chalk, only to have him have a bad skin reaction to it, and that could happen with any horse, particularly if you are using different pigments.
As an alternative, you could use a dark coloured rug and paint/chalk on it, or there's a video on the market (I forget the name) where someone has painted up a grey, which would be easier for you.
S
these are done with kids poster paint i buy in bulk off the internet!
washes off your clothes nicely 2!
I find chalk a pain,its not as clear and blows off especially when students touch it!