Another creature on The Revenge (the ship in Expedition to the Barrier Peaks). The horse cactus pineto. Converted from 7th edition Gamma World (4e version). I still think using 4e monster design philosophy with 5e rules produces much better monsters than the straight up 5e rules.
PINETO (Horse Cactus)
Large plant, unaligned
Armour Class 13 (natural armour)
Hit Points 68 (8d10 +24)
Speed 60 ft. plus galloping cactus
STR 18 (+4) DEX 15 (+2) CON 16 (+3)
INT 2 (-4) WIS 13 (+1) CHA 15 (+2)
Damage Resistance piercing from nonmagical weapons
Damage Immunities lightning, poison
Condition Immunities charmed, exhaustion, frightened, poisoned
Damage Vulnerabilities fire
Senses passive Perception 11
Challenge 3 (700 XP)
Painful Barb. Any creature that strikes the pineto in melee also takes 5 (2d10) poison damage once per attack.
Galloping Cactus. The first time it moves adjacent to a creature it inflicts 5 (2d10) piercing damage from the bristling spikes that covers its body. This does not provoke attacks of opportunity.
Tail Slap. Melee Weapon Attack: +6 to hit, reach 5 ft., one target.
Hit: 13 (2d8 + 4) bludgeoning damage plus 7 (2d6) poison damage.
Thorn Barrage (Recharge 5-6}. The pineto shoots thorns in a 30-foot line that is 5 feet wide. Each creature in that line must make a DC 12 Dexterity saving throw, taking 22 (4d10) poison damage on a failed save, or half as much damage on a successful one. Failure also indicates the creature is slowed as the spell.
These oddly mutated plants have a horizontal trunk and movable branches, adapted to rapid locomotion. A large clump of roots and keen visual and olfactory appendages at the base of its trunk vaguely resembles a shaggy horse’s head. It feeds on nutrimental matter, which its tendrils thrust into the trunk cavity where it is absorbed by thousands of tiny rootlets. The Pineto takes on moisture much like an animal, by dipping its “head” (root clump) into a stream or pond.
Pinetos move about quickly in order to avoid predators. When captured, the Pineto can be ridden or used as a beast of burden (carrying up to 800kg/1800lbs on their backs). Control is by means of a sharp goad which is jammed into the Pineto, just behind the root clump. Using this device, the creature can be made to move, run, turn, and stop at a rider’s command. Note, however, that the Pineto is covered with a dense growth of sharp needles, and even the rider may suffer an accidental attack from its painful barbs.