load bank: 12V, 120 A
You'll need a load that automatically adapts to different source voltages, so that the power converted in that load to heat is constant.

Typically, you'd implement something like that using transistors, which you control accordingly.

So, yes, you'll need some form of controller; a feedback loop seems highly desirable.

In this scenario, it's probably easiest to use a set of MOSFETs as the load "resistors". Since the whole math is really easy,

$$P =U\cdot I\text,$$

you can basically just write a loop that decreases the effective resistance of your MOSFETs (by increasing the gate voltage in an N-channel mosfet, typically) until you hit your "should be" power point. Of course, having a fully fledged PID controller is a much better approach, avoiding oscillations, but this all comes down to how much faster you can control the MOSFET than your power source is going to change. 

Now, practically:


12 V · 120 A = 1.44 kW. You need to get this frigging amount of heat out of your transistors in order for them to not burn. Good luck with that. 
That's still 180 W per MOSFET if you have 8.
To allow 12 V to push 120 A through a system, that system needs to have an effective resistance of 0.1 Ω. That makes finding MOSFETs that can both do those currents and an Rds_on low enough to fulfill that expensive.
A PCB trace designed to carry 120 A over 10 cm, not allowed to get more than 40 °C hotter by itself (not counting hot transistors) of 70µm copper (twice what you find on most PCB) would have to be 4.8cm wide. That's a layout challenge right there; the narrower the traces, the higher the inherent resistance (making the previous point harder to achieve)
Please consider the voltage spikes that a few nH of parasitic inductivity would induce if you suddenly turn of the 120 A (e.g. for automatic shutdown in case of overtemperature). Not pretty, at all.
120 A really sounds like you should an expert design this stuff. The amount of sharp stuff that might explode and fly around is really not to be underestimated at these currents.