What regions do Red Beacons/Supply Caches usually appear in?
Is it just random/rare?
The location of the beacons is fixed and each position has a range of colors the beacon can be. Not all colors are possible everywhere, see this map for the possible colors at each location. Higher quality drops are rarer than lower quality drops, though, so a location with a red-mark does mostly spawn non-red drops. Some drops also trigger the spawning of creatures like a rex in the area.
According to the gamepedia entry for ARK, beacons have set spawn locations but any color can spawn at a given beacon spawn location. You're going to have to rely on luck if you're gunning for a specific color.