Hmm, arugula, chicory, endive, radicchio, puntarella, mustard greens, lacinato kale, broccoli rabe, dandelions, all taste bitter all are good, plus I'm sure there are others. oh yeah artichokes!!!!
@OP post a picture up, we might be able to id it that way. But rhubarb is singularly definitive in taste, you won't be able to confuse it with anything else. so if it's not super tart, with zincy undertones, it's probably not rhubarb