Why do you suppose that race horses feel the need to piss with such notable urgency?

That phrase must come from the loudness of a horse's urination. Their streams start from pretty high up from the ground, so there's a lot more noise... Also, a lot of urine comes from a horse... So it seems they must've had to go, like when a human has to go, the stream is loud and plentiful... I'm assuming these are the reasons for where that common phrase originates. And "Why racehorse?" Maybe that's the only exposure to horses a lot of people had.