When you say the roots looked normal, what did they look like? What about the rhizomes? How often does your ZZ get watered? Does that pot it is in have drainage?

To me it looks like they rotted at the base which indicates too much water, but more info would help.

I'd say I'm even more sure about my theory. It's definitely a mammillaria. Monkey tail have long, almost fuzzy spikes and are thinner, hence the name. Rat tail cactus look more similar to yours, but again, they are very long and skinny, like, well, a rat tail lol.

I don't this this is a monkey tail cactus. I think it might be a mammillaria, specifically mammillaria pilcayensis. They often grow up and then begin to grow prostrate as they lengthen.