One from the 17th century. Any clues?

I Ride without a Saddle on a thing
That has no legs nor bones, yet thus straddling,
Am thereon stately mounted, to the end
I may two friends (that are decay'd) befriend.
Wherefore, when I before them do appear,
That which to them seems doubtful, I make clear.
Now when this business I have thus begun,
I'll not dismounted be till I have done:
Then shall I to my Lodging be convey'd,
Till I in this kinde am again employ'd.