The single largest penis you've ever seen appears before you. Its considerable girth is braced and reinforced by forty-five degree two-by-fours on either side. Terra, your trusty steed has not yet faced a dick this formidable; you can feel her trepidation in the reins.
While still in full gallop, you remove your left hand and slide it gently, reassuringly down her muscled neck then lean forward until your lips are near her upright ears.
"Fear not, my friend," you instruct her. "Though this cock casts a longer, firmer, more lasting shadow than any that've come before, trust in your training and you may tackle his veiny shaft yet!"
You dig your heels into Terra's side and, filled with an almost tangible new confidence, she speed toward the pointing, accusatory phallus.
"Do it, girl!" you cheer. "Take that dick!"
Your deeds that day, and those of your valiant mare, are immortalized on bumper stickers for years to come.