Large beast, unaligned
Armor Class: 14 + PB (natural armour)
Hit Points: 5 times your barbarian level (the beast has a number of Hit Dice [d8s] equal to your barbarian level)
Speed:
40 ft
Saving Throws: Strength +STR + PB, Dexterity +2 + PB
Skills: Athletics +STR + PB, Perception +2 + PB
Senses: Darkvision 60 ft., passive Perception 12 + PB
Languages: Understands the languages you speak
Proficiency Bonus: Equals your proficiency bonus
Shared Strength. The Thunderbeast has a strength score equal to your Strength score.
Ambush Adept. When the Thunderbeast hits a creature that is surprised, or when it scores a critical hit, the creature is grappled or knocked prone (your choice).
Actions
Bite. Melee Weapon Attack: +STR + PB to hit, reach 5 ft., one target. Hit: 1d6 + PB piercing damage.