Old Woolly Rupert Medium Aberration, true neutral ? 0 xp

Armor class 25 (natural)
Hit points
Speed

STR 8 (-1)
DEX 26 (+8)
CON 14 (+2)
INT 10 (0)
WIS 10 (0)
CHA 16 (+3)

Save Throws: Dex +14, Int +6, Wis +6, Cha +9

Damage Resistances: psychic, bludgeoning, piercing and slashing from magical weapons

Damage Immunities: all mundane physical damage

Condition Immunities: charmed, frightened, paralyzed, restrained, prone, stunned

Senses: darkvision 120 ft.

Languages: Telepathy (500 ft.)

Challenge: ? (0 xp)

Ethereal Jaunt: As a bonus action, Old Woolly can
magically shift from the Material Plane to the Ethereal
Plane, or vice versa.

Shapechanger. Old Woolly can change his size at will, to any size from Tiny to Gargantuan.

Avoidance. If Old Woolly is subjected to an effect that
allows it to make a saving throw to take only half damage, it instead takes no damage if it succeeds on the saving throw, and only half damage if it fails.

Legendary Resistance (3/Day). If Old Woolly fails a
saving throw, it can choose to succeed instead.

Actions

Multiattack. The hamster can make three melee attacks
with either its claws or bite.

Bite. Melee Weapon Attack: +14 to hit, reach 5 ft., one
target. Hit: 11 (1d10 + 8) piercing damage. The target
must make a Strength saving throw (DC 14) or be
grappled. While the target is grappled, the hamster
cannot use this attack on any other creatures.

Claws. Melee Weapon Attack: +4 to hit, reach 5 ft., one
target. Hit: 9 (1d6+8) slashing damage.

Fire Breath (Recharge 1d2): The hamster exhales fire in a 15-foot cone. Each creature in that area must make a DC 20 Dexterity saving throw, taking 48 (14d6) fire
damage on a failed save, or half as much damage on a
successful one. Creatures grappled by the hamster fail
this save automatically.

Legendary Actions

As a legendary action (no limit), Old Woolly Rupert can change his available ability or use a passive one. He can use any ability he has seen a creature use previously. To start, he already knows the following abilities: Fly, dragon breathe (fire), psyonics, invisibility, phase, spider climb, multiple heads (hydra).