Flavor Country (Chad Wolanski)
Talk to Chad Wolanski, south of the FANG center.
'Talk' might be over-glamorizing what happens here, as neither the Deputy nor Chad actually possess the power of speech, but the exchange ends with a quest to run over things that have antlers.
Note that you don't actually have to be on the road. A good spot is a bit of marshy land south and slightly east of the FANG Center, northwest of Breakthrough Camp.
This area is teeming with horned ungulates, and ground is flat enough to drive on. Your choice of vehicle is between the manuverable ATV which may take several hits to kill a target, and a truck, which is a single-hitter, but less nimble.
Do be careful of driving out into the water itself. If you're having trouble catching them on the flats, try to herd or scare the beasts south onto the road, for easier smashing.
Remember to stop and loot the corpse as soon as possible- get too far away and it will despawn.
Regardless of your perks, you'll still need to take down & loot four individual beasts to meet the mission requirement. When you have, return to Chad to complete the mission.