Technically, this is an alien creature so it sits under the Xenos category. Though it could quite happily live beside the other magical beasts wrought by mad wizards like bullette and owlbear.
Large monstrosity, unaligned
Armour Class 18 (natural armour)
Hit Points 218 (19d10+114)
Speed 50 ft., burrow 30 ft.
STR 19 (+4) DEX 11 (+0) CON 21 (+5)
INT 2 (-4) WIS 11 (+0) CHA 5 (-3)
Skills Perception +6
Senses darkvision 60ft., tremorsense 60ft., passive Perception 16
Challenge 14 (11,500 XP)
Ambush. When a feranth charges, instead of making a gore attack at the end of the charge, it can instead use its skull-splitting roar ability, although the DC to resist this ability is 4 lower when the ability is used in this fashion.
Charge. If the feranth moves at least 15 feet straight toward a target and then hits it with a gore attack on the same turn, the target takes an extra 38 (7d10) piercing damage. If the target is a creature, it must succeed on a DC 21 Strength saving throw or be pushed up to 20 feet away and knocked prone.
Burrowing Charge. A feranth can use the charge and dash actions while burrowing.
Hardened Body. A feranth’s natural attacks count as adamantine for the purpose of overcoming damage reduction and hardness.
Bite. Melee Weapon Attack: +9 to hit, reach 5 ft., one target.
Hit: 30 (4d12 + 4) piercing damage.
Gore. Melee Weapon Attack: +9 to hit, reach 10ft., one target.
Hit: 42 (7d10 +4) piercing damage.
Skull-Splitting Roar (Recharges on a 5 or 6). A feranth can unleash a terrible roar, affecting all creatures in a 15-foot cone. Any creatures in the area of the cone must succeed at a DC 24 Constitution save or be dazed (disadvantage on attack rolls, ½ movement) and deafened for 1 minute.
Quick Strike. Whenever a feranth successfully dazes a creature using its skull-splitting roar ability, it can make an attack of opportunity against that creature.
A droning hum announces the presence of this enormous horned beast. Powerful muscles ripple just beneath its mottled black hide.