Ichneumon

With powerful jaws and teeth capable of shearing scales from meat with ease, the creature resembles a weasel the size of a brown bear.

Outsized Temerity. These creatures live in burrows and caves whose previous inhabitants were likely devoured by the new residents. Wily hunters, ichneumons prey upon all kinds of creatures, but they go to great lengths and far out of their way to feast on dragon eggs, or the dragons themselves if they can.

Resourceful Rodent. Ichneumons have a keen animal intelligence, and they make an innovative use of their environment. A musky oil with insulating properties coats the fur of an ichneumon, granting it a measure of protection. When an ichneumon has the luxury of time to prepare before hunting, it finds the nearest source of mud and applies layer after layer to itself. The mud bonds with the creature's oily coat and forms a hard, thick outer armor, enhancing its natural protections.

Hunted and Hated. The ichneumon's very existence is a source of humiliation and outrage for dragonkind. The mention of a dragon's name who was killed by an ichneumon is considered distasteful among dragons once they have hunted for the killer and settled the matter. Different standards of decency apply for other beings, however, and any non-dragon foolish enough to mention an ichneumon within hearing of a dragon might never be heard from again.

5e SRD

Eight-legged beast the size of a pony with a bear-like head and long furry coat

The Ichneumon is an eight-legged beast the size of a pony. Its head is like that of a bear. It has a long furry coat that can be white, black, red, blue, or green. It has a long, thick tapered tail. The male also has a lion mane and is up to 25% larger (and has 15 HD), and can weigh in excess of a thousand pounds. The ichneumon loves to eat dragon scales and because of this is also known as the "Dragon Eater". When encountered in the wild, it will have no treasure. A female will usually nest in the lair of a defeated dragon along with a kit of up to HD6 and the dragon's hoard. Males, in contrast, lair in small isolated dens with no treasure.

Basic Fantasy Field Guide Omnibus

Amphisbaena	Antelope	Archaeopteryx	Archelon	Army Ant Swarm
Aurumvorax	Axe Beak	Baboon	Badger	Baluchitherium
Bat	Bat Swarm	Behemoth Hippopotamus	Blindheim	Blue Whale
Boar	Bulette	Bunyip	Cameroceras	Cat
Cat Lord	Cave Bear	Cave Fisher	Cave Lion	Centipede Swarm
Cheetah	Cockroach Swarm	Constrictor Snake	Crocodile	Devilhunter Behemoth
Devilsnout Behemoth	Digmaul	Dire Ape	Dire Badger	Dire Bat
Dire Boar	Dire Crocodile	Dire Polar Bear	Dire Rat	Dire Weasel
Dire Wolf	Dire Wolverine	Dodo	Dog	Dolphin
Eagle	Electric Eel	Elephant	Elk	Emperor Walrus
Foo	Foo Dog	Foo Lion	Frog Father	Frost Worm
Gar	Giant Ant	Giant Centipede	Giant Chameleon	Giant Cockroach
Giant Crab	Giant Dragonfly	Giant Dragonfly Nymph	Giant Eagle	Giant Flea
Giant Fly	Giant Frog	Giant Gar	Giant Gecko	Giant Jellyfish
Giant Leech	Giant Maggot	Giant Mantis	Giant Moray Eel	Giant Mosquito
Giant Octopus	Giant Owl	Giant Porcupine	Giant Scorpion	Giant Seahorse
Giant Skunk	Giant Snapping Turtle	Giant Solifugid	Giant Spider	Giant Squid
Giant Stag Beetle	Giant Tick	Giant Toad	Giant Tortoise	Giant Vulture
Giant Wasp	Giant Weasel	Goblin Dog	Goliath Frog	Gorilla
Gorthek	Great Horned Owl	Great White Whale	Griffon	Grizzly Bear
Hawk	Hippocampus	Hippogriff	hippopotamus	Hodag
Horse	Hyaenodon	Hyena	Immense Tortoise	Kangaroo
Killer Seahorse	Kongamato	Leopard	Lion	Makara Vahana
Manta Ray	Mastodon	Megalania	Megalodon	Megaprimatus
Megawhale	Monkey	Monkey Swarm	Narwhal	Octopus
Orca	Owl	Parasaurolophus	Platypus	Poison Frog
Polar Bear	Pony	Porcupine	Purple Worm	Rat
Rat Swarm	Raven	Rhinoceros	Roc	Sea Serpent
Seawolf Behemoth	Shark	Shocker Lizard	Skunk	Sloth
Slurk	Smilodon	Snake Swarm	Snapping Turtle	Spider Swarm
Squid	Stingray	Therizinosaurus	Thylacine	Tick Swarm
Tiger	Toad	Toy Behemoth	Trollhound	Troodon
Trumpeter Swan	Tuatara	Tylosaur	Tylosaurus	Venomous Snake
Venomous Snake Swarm	Viper	Vulture	Walrus	Wasp Swarm
Weasel	Whale	Wolf	Wolliped	Wolverine
Wooly Rhinoceros	Yzobu