I am called X Y Z, and I am supposed to represent the unknown quantity which is left

I am a horse, belonging to a circle of friends, who have resolved to bear you on the back, at the rate of eight miles a figure-circle, and to deliver you safely within the limits…

I am a member of the body politic, called the state of England, wherein I was first created, and wherein I was long kept, and wherein I have been long detained.…