Yes, the crap on the horses is to drive off biting insects.
The guy in the middle is leaning on the handle of a pump. The elbow on the top of the pump might be an outlet, but could also be a vent so what I suspect is double pistons, can move freely.
The galvanized container to the right of the kid, looks like a potable water container for people.
If there's a water tank in the wagon, it can't be very big, with only two horses... even in Kansas.
Maybe it's just a pump, possibly with some hoses, to go where they need some water moved, like stock tanks or irrigation ditches.