Prepaid hotspots

Am I able to purchase a prepaid hotspot for use in a single month?

Yes.

 I wouldn't need it otherwise the rest of the year.  Are these devices connected to any type of contract? 

No.

Or you simply buy data for it as you need it?

Yes but your prepaid account will get closed if you go more than a year without buying data. When you buy a data plan, whatever amount of data you buy is good for 30 days or until used up.