Critters

The wasteland does not lack for things that want to kill you. Zombies get the reputation, and Raiders get the fear, but the mutated fauna that roam between settlements account for more deaths on the road than most survivors care to admit. They are not undead. They are not human. They are animals, changed by the Infection and the broken world into something that does not fit neatly into any pre-Fall taxonomy. Settlers call them critters, and the word carries more weight than it should.
  Every critter in the wastes can be understood through three dimensions: Scale, Scope, and Environment. Scale measures how dangerous the creature is, from the Nuisance-tier pests that scatter when you shout at them to the Apocalyptic-tier horrors that require every survivor in a settlement to mobilize or die trying. Scope defines the creature's behavioral archetype, the particular way it has adapted to threaten the living. Armored critters tank damage behind reinforced hides. Carnivorous critters hunt with relentless hunger. Psionic critters attack the mind. Ten Scope types exist in total, and each changes the rules of engagement. Environment determines where the creature thrives: aquatic depths, irradiated wastelands, fungal overgrowths, urban ruins, dense forests, or the Mortis Amaranthine itself. Combine one option from each dimension and you have over 250 unique creature profiles, each of which can be skinned as any mutated animal the wastes might produce.
  The five threat tiers follow a pattern that veteran survivors learn to read at a distance. Tier 1 Nuisances are small, fast, and disposable; they move at a run or sprint and carry only ten points of Body. Tier 2 Hazards demand attention but remain within local control. Tier 3 Menaces require organized response and specialized tactics. Tier 4 Catastrophic threats can devastate entire communities and demand coordinated mobilization. Tier 5 Apocalyptic creatures are extinction-level events: 250 Body, massive damage output, and movement so slow the ground shakes with each step. There is an inverse relationship between size and speed that mirrors the pattern seen in zombies. The smaller the critter, the faster it moves. The larger it grows, the harder each step hits. A Nuisance-tier critter will outrun you. An Apocalyptic-tier critter does not need to.
  One thing every newcomer needs to understand: critters are not pets. They are not domesticable. They will not bond with you, follow your commands, or guard your camp. They will tear you apart, gore you, or eat you. Pets exist only in the stories survivors tell about their downtime, never in the field. Only NPC factions like Raiders have been observed using critter companions during live encounters, and that should tell you everything about the kind of people who try.