Herds of Eevee

Typical composition of the herd
A mixed herd usually has between 8–20 individuals, with the following approximate distribution (highly variable depending on the habitat):
Eevee (base form): 40–60% of the herd (juveniles and those that have not yet decided to evolve or have not found the appropriate stimulus). They are the social and reproductive "core".
Vaporeon and Leafeon: The most common among the evolutions (both adapt very well to environments with water or abundant vegetation).
Jolteon and Flareon: Less frequent, they usually appear in areas with storms or volcanic/dry areas.
Espeon and Umbreon: Rare in mixed day/night herds; sometimes form separate subgroups (day vs. night).
Glaceon: Only in cold or mountainous climates (unlikely in temperate herds).
Sylveon: The rarest in the wild; tends to appear when there is a strong emotional bond (with young or with close humans).
In large herds there are sometimes 1-2 "alphas" (usually a mature Espeon or Umbreon due to their intelligence and perception) who coordinate the group.

Habits and behavior
Social and familial: Contrary to what the Pokédex states (that many Eeveelutions are "solitary"), in mixed groups they exhibit pack-like behavior similar to wolves or foxes, with a strong, albeit soft, hierarchy. Young Eevee constantly play, and adult evolutions protect and teach them.
Division of roles by type (widely accepted headcanon):
Vaporeon → Aquatic explorers, fishermen, water source keepers.
Jolteon → Fast Sentinels, Perimeter Patrols (their speed and electricity scare away threats).
Flareon → Warms up the group on cold nights, cooks (burn hard berries to soften them).
Leafeon → Main foragers, they know medicinal plants and berries.
Glaceon → They keep food fresh, they help in warm climates with cooling.
Espeon → Strategists, predict dangers (with their solar/psychic perception).
Umbreon → Night watchmen, stealthy hunters.
Sylveon → Emotional caregiver, calms disputes with his ribbons and fairy aura (the "healer" of the group).
Hunting and feeding: They hunt in a coordinated team. Jolteon stun prey, Umbreon ambush them, and Vaporeon drag them into the water. They eat berries, large insects, small mammals, and fish. Young Eevee eat more soft berries.
Territoriality: They mark their territory with scents (Flareon and Umbreon leave strong marks). They are curious about humans, but flee if they feel threatened (except for Sylveon, which sometimes approaches).
Day/Night Cycle: During the day, Espeon + Leafeon + Vaporeon dominate. At night, Umbreon + Jolteon + Glaceon take control.

General appearance of the pack
Imagine a colorful and diverse group moving through a forest, meadow, or mountainside:
The soft, creamy brown of the Eevee dominates (many pups jumping and rolling).
Bright blue flashes (Vaporeon), leaf greens (Leafeon), electric yellows (Jolteon), intense reds (Flareon).
Elegant purples (Espeon) walking with haughty posture.
Blacks with luminous rings (Umbreon) that glow faintly at sunset.
Crystalline ice blues (Glaceon) leaving icy footprints.
White/pink ones with waving ribbons (Sylveon) that seem to float among the others.
The herd moves in a loose but coordinated formation: the Eevee in the center, protected by the larger, stronger Eevee on the edges. When resting, they form a large circle, with the Flareon and Glaceon in the middle to regulate temperature, and Umbreon/Sylveon guarding the edges.