What the fuck is a Witch Engine

It clearly has something to do with Phyrexia, but it hasn't been errata'd to be Phyrexian - it's just a Horror.

But they sure are in the background of more than I've ever realized before! Are they like, native creatures of Phyrexia? Are they exactly what they sound like, Horrors created by witchcraft for witchy purposes?

I don't like how much they are in the background of things I haven't noticed before. It upsets me.