A horse's head going down the drain - When you’re so sure something will fail it might as well be

When you’re so sure something will fail it might as well be a horse’s head getting flushed.

She said she’d be rich by 25. I said, 'That’s a horse’s head going down the drain.'

He bet his last dollar on a lottery ticket. I said, 'That’s a horse’s head going down the drain.'

My dog tried to eat my homework. I said, 'That’s a horse’s head going down the drain.'