There are five known Freaks of Nature which appear to be larger, stronger, and more deadly versions of their smaller infected equivalents.
Thor is a Goon type infected located in the north east of the countryside map. Look for the end of a highway and a large wall with barbed wire covering the upper edge, he is located on the other side of this wall. A simple way to get access is jump around either side of the wall and use the grappling hook to move towards the area. Located in the same place as Thor is a mysterious box that contains the USS Techland paint job and outfit.
Holler is a larger and stronger than average Demolisher infected and can be found west of Thor, north of the water pumping station, and north west of Jasir's farm. Holler has more armor than a typical Demolisher and as such can take significantly more damage before dying. Once killed the area he was trapped in becomes a safe zone and NPCs return to the buildings — the Heavy Venom blueprint can be found within the safe zone once killed.
David & Goliath
David and Goliath are a Viral and Biter duo located to the south east of the main island in The Countryside. The easiest way to reach the island is to drive your buggy to the military outpost near the wall in the south east, then run as far east as possible past the wall and swim the remaining distance. Also found on the same island is the Zig Zag buggy paint job and The Depleter blueprint.
Behemoth is the second Demolisher Freak of Nature and can be found inside a factory within the industrial area towards the south east of the map. In the same area as Behemoth is the Big Thunder blueprint. Behemoth is also a bit stronger than Holler. In addition to that, the hangar he is in has explosive barrels scattered around which can be used to make quick work of him, if used correctly.
Beelzebufo is an over-sized Toad trapped inside a potential safe zone that looks similar to Holler, located west of Behemoth just across the lake. Due to the highly damaging ranged attack this Freak of Nature can be one of the most dangerous to fight, especially on higher difficulties. One viable strategy is to use thrown weapons or firearms to weaken, or kill, from a distance and use a highly damaging weapon to deliver the killing blows. Alternatively, the crossbow can be used to snipe him in the head from a safe range atop a vehicle outside his fenced in area, which prevents him from attacking and does not attract Virals. The Big Swing blueprint can be found in the same area as Beelzebufo once killed.