That's interesting DBA. I spent a lot of time as a kid at the track. I come from a family that takes playing the ponies very seriously (my grandfather came over the pond and settled in the Bronx and, to my grandmother's continued consternation, he played the horses daily. Unfortunately, most of his horses are still running. One day his luck changed and he came home with a paper bag full of money after hitting a huge daily double. My grandmother took the bag, jumped on a bus to Atlantic City, and bought 20 acres of land in the Pine Barrens. My grandfather was furious, but when he learned Atlantic City had a track that ran both the flats and trotters he came around).

I always thought "piss like a racehorse" was related to horses being more comfortable urinating in their own pens as opposed to urinating on the track, in show situations, etc. and that after their return to their pens they would release the great smoking stream they had been holding while occupied elsewhere.
Thanks for the information. I take great pleasure in learning quirky new things.