Does anybody know of a rear disc brake adapter for a bike with horizontal dropouts? I have an older bike that has V brakes and no disc tabs. I may have to make one, but I thought I would ask first...

I dont see any way short of hiring a machine shop and custom manufacture needs to be done.

you now want disc brakes? go buy a new bike thus equipped.

If you really want to convert it then send a photo of your dropout and rear triangle to Therapy Components and see if they'd might be able to recommend something for you, they make brake adapters and floating brake kits that might work on your frame but it won't be cheap.

https://2btherapy.com/contact

Most of your braking should be done with the front wheel so that'd be where you'd see the most benefit from a disc brake, there's nothing wrong with having a disc on the front and a v-brake on the rear.

Personally I wouldn't bother trying to convert it.