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You could use an aquarium heater possibly, but I guess that would be all relative to how cold it actually gets and you would have to built some kind of protective device. There are also heaters available at livestock supply places that are designed for this kind of thing.
What is the purpose of the filter? Are they designer Horses or something? Will this system be located in an area where the water can freeze?
As for water level you can buy an inexpensive float valve from a plumbing place (float valve from a humidifier) and you won't need any circuitry to get it working, it fills automatically.
Have a look at some washing machine circuits and info it may give you some insight:
I live in the cold too, it's been -25 for the last week.... Brrrrrr..
The reason I questions the freezing bit, is that I did not know if these critters were in a heated stable or not and if you just wanted the water a little warmer.
And since their water will be in a place where it will freeze you have to consider the water supply line which will also freeze. You will have to have a couple of solenoid valves. One to open the water supply and one to drain the line after or have the supply line wrapped in heat tape.
I would recommend something like this for the beasts water: