A horse's head going down the drain - When something is so stupid it might as well be tossed into

When something is so stupid it might as well be tossed into the sewer with a horse's head.

I told my mom I’d pass the test. She said, 'A horse's head going down the drain.'

My friend tried to convince me aliens are real. I said, 'That’s a horse's head going down the drain.'

He asked for a raise. His boss said, 'You’re going down the drain with a horse's head.'