Megafauna, Arsinoitherium, "Primeval Juggernaut Of The Mire" | Fantasy I Sci-Fi I Books I Films I World Building

Megafauna, Arsinoitherium, “Primeval Juggernaut of the Mire”

“Towering like a living battering ram, the horned Arsinoitherium crashes through swamps and forests—an unstoppable relic of a forgotten world!”

The Arsinoitherium resembles a bulkier, more primeval rhinoceros with a powerful barrel chest, column-like legs, and a thick, leathery hide often caked with mud or swamp moss. Its most striking feature is the pair of enormous, forward-curving horns—fused bone extensions that jut from its snout like the prow of a siege ram. Smaller hornlets sometimes stud the base of its nasal ridge, giving it an armored, ceremonial look. Its eyes are small and recessed, offering little expression beyond the dull patience of a behemoth that knows no fear.

Behaviour:
Arsinoitherium is a solitary and slow-moving herbivore, spending much of its time foraging or resting. Despite its bulk, it can charge with surprising speed and force when startled or challenged. It is not inherently aggressive but is deeply territorial, especially around water sources or nesting grounds. Vocalizations are deep and resonant—low rumbles that vibrate through the air and water, signaling warnings to rivals or offspring.

Habitat:
Native to warm, prehistoric floodplains and mangrove-choked swamps, Arsinoitherium prefers shallow, soft-bottomed basins where its great weight won’t bog it down. In fantasy settings, it is often found in primeval lowlands, hidden jungle valleys, Hollow Earth oases, or magical wetlands untouched by time. It avoids steep terrain or dry scrublands, relying on access to water and leafy vegetation.

Modus Operandi:
Arsinoitherium is a defensive powerhouse. When threatened, it lowers its head and bellows before launching into a devastating horn charge—capable of toppling trees or flipping carts. It uses its horns not only for defense but for dominance displays during mating or territory disputes. Its thick hide makes it highly resistant to arrows and small blades, and it often tries to trample or slam opponents into submission rather than engage in drawn-out battles.

Motivation:
Driven primarily by instinct, Arsinoitherium seeks security, food, and territory. However, in some ancient or enchanted regions, a few may have been bonded to primeval druids, fey courts, or guardian spirits as living relics of an older age. Such individuals may defend ancient groves or be awakened by planar shifts that threaten their hidden domain.

Arsinoitherium 5e
Arsinoitherium, Pathfinder

Arsinoitherium

Large Beast, Unaligned
“Primeval Juggernaut of the Mire”

Challenge: 4 (1,100 XP) Proficiency Bonus: +2
Armor Class: 15 (thick hide)
Hit Points: 95 (10d10 + 40)
Speed: 40 ft.

STR	DEX	CON	INT	WIS	CHA
20 (+5)	9 (–1)	18 (+4)	2 (–4)	12 (+1)	6 (–2)

Saving Throws

Con +6, Wis +3

Skills

Perception +3

Senses

Passive Perception 13

Languages

—

Proficiency Bonus

Traits

Relic Bulk

The Arsinoitherium is immune to being knocked prone by creatures smaller than Huge.

Sure-Footed

The Arsinoitherium has advantage on Strength and Dexterity saving throws made to resist being moved or knocked prone while standing on solid ground.

Swamp Charger

If the Arsinoitherium moves at least 20 feet straight toward a creature and hits it with a Horn Slam on the same turn, the target takes an extra 13 (2d12) bludgeoning damage and must succeed on a DC 15 Strength saving throw or be knocked prone.

Actions

Multiattack

The Arsinoitherium makes two Horn Slam attacks.

Horn Slam

Melee Weapon Attack: +7 to hit, reach 5 ft., one target
Hit: 17 (2d10 + 5) bludgeoning damage

Thunderous Charge (Recharge 5–6)

The Arsinoitherium bellows and charges up to 40 feet in a straight line, passing through up to two Medium or smaller creatures. Each must succeed on a DC 15 Strength saving throw or take 22 (4d8 + 4) bludgeoning damage and be knocked prone. On a success, a creature takes half damage and isn’t knocked prone.
The charge ends if it hits a creature that resists being moved or a solid barrier.

Tactics

Territorial Brute: The Arsinoitherium aggressively defends its territory, especially from loud or fast-moving intruders.
Opening Shock: It often begins combat with Thunderous Charge to scatter or knock down threats.
Control Beast: It blocks narrow swamp paths, charging any who attempt to pass without regard for tactics beyond brute force.
Stands Its Ground: The beast rarely retreats unless grievously wounded, relying on mass and intimidation to dominate.

Arsinoitherium

This creature is much like a rhino, some six feet tall and ten feet long, with a pair of massive, knife-like horns curving up from its nose.

The arsinoitherium is a herbivore, but it displays a fierce and threatening behavior when it perceives danger: bellowing, stamping, and tossing its head. Threats that do not recede from such warnings are met with a fierce bellow and a charge.

Arsinoitherium CR 7

XP 3,200
N Large animal
Init +0; Senses low-light vision, scent; Perception +13

DEFENSE

AC 20, touch 9, flat-footed 20 (+11 natural, –1 size)
hp 85 (9d8+45)
Fort +13, Ref +6, Will +4

OFFENSE

Speed 30 ft.
Melee gore +14 (4d8+13)
Space 10 ft.; Reach 5 ft.
Special Attacks powerful charge (4d8+13), trample (2d8+13, DC 23)

STATISTICS

Str 28, Dex 10, Con 21, Int 2, Wis 13, Cha 3
Base Atk +6; CMB +16; CMD 26 (30 vs. trip)
Feats Diehard, Endurance, Great Fortitude, Improved Overrun, Power Attack
Skills Perception +13

ECOLOGY

Environment temperate plains
Organization solitary, pair, or herd (3–12)
Treasure none

Arsinoitherium Companions

Starting Statistics: Size Medium; Speed 30 ft.; AC +4 natural; Attack gore (1d8); Ability Scores Str 14, Dex 12, Con 15, Int 2, Wis 13, Cha 3; Special Qualities low-light vision, scent.

7th-Level Advancement: Size Large; AC +2 natural; Attack gore (2d8); Ability Scores Str +8, Dex –2, Con +4; Special Qualities powerful charge (2d8), trample.

Section 15: Copyright Notice

Pathfinder Roleplaying Game Bestiary 2, © 2010, Paizo Publishing, LLC; Authors Wolfgang Baur, Jason Bulmahn, Adam Daigle, Graeme Davis, Crystal Frasier, Joshua J. Frost, Tim Hitchcock, Brandon Hodge, James Jacobs, Steve Kenson, Hal MacLean, Martin Mason, Rob McCreary, Erik Mona, Jason Nelson, Patrick Renie, Sean K Reynolds, F. Wesley Schneider, Owen K.C. Stephens, James L. Sutter, Russ Taylor, and Greg A. Vaughan, based on material by Jonathan Tweet, Monte Cook, and Skip Williams.

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