Hot tubs are another popular option and there is no reason you can't use the same tank as your pool, although the big round tanks would take a lot of heating. This is one of our 5ft oval stock tanks with a fired heater coil produced by our friends at Firetree Metal Works.
Alex at Firetree makes these coils and he can help with the necessary bulkhead connectors and hoses. We are very happy to drill the holes required if you would like us to.
We have the 1 1/4 inch (32mm) heater coil connected to our 5ft oval tank. We chose this option as the coil has a larger internal area and therefore heats the tank faster. Also, the hoses which are supplied with many above ground pool pumps are also 32mm. This now gives us the option to disconnect the heater coil and connect a filter pump to the same "bulk head connectors" when we are not heating the tank.