These aren't the only things out there of course, but it should get you started and keep your drawing busy. Available in smaller, more digestible chunks at www.htdhorses.com :
Horse humor cartoon. Horse afraid of the water puddle.
Wanna make a sign that says this to put over one of the horse stalls
...you whispered into my heart... and my life changed forever More
It is our duty to our horses to kindly show them what we want and if what we want requires us to hurt them in any way then we ought not to ask.
AMEN...you wouldnt believe the people that come up to me and say riding horses is not a sport..they need to read this.
Ride aggressive, be confident because it's all or nothing, and the number one thing is you cannot ride timid! ~Charmayne James
My horse looks heartbroken if I even look at another horse
http://fourfaults.tumblr.com/ or ride with no stirrups or jump or ride through a buck/rear
Well played horses, well played.