Scalamanx
Image © Working Partners Ltd, accessed at the Beast Quest wiki here
[Sponsored by @glarnboudin. Beast Quest is a series of middle grade books that started publishing when I was in undergrad, but if they were around when I was in the target demographic, I probably would have imprinted on them hard. Each one is a monster of the week affair, with the Beasts being fought or rescued by earnest kid adventurers fighting against various evil wizards. The Beasts, even the evil ones, often have some sort of role in the ecology of the world. The post sponsor described it to me as "Harryhausen meets the Legendary Monsterverse", which I would have gobbled up as a kid. Although Scamandrax exists in universe kind of in a vacuum (he was slain by the protagonist's mother and is accidentally resurrected to wreak havoc in the modern era), I went ahead and added an ecological role for it that seemed to fit its general vibe.]
Scalamanx CR 12 LE Magical Beast This creature appears as a lizard the size of a wagon, its hide made of rocky scales that glow from an internal heat. Its eyes are large and forward facing, and ridges of tissue like those of a newt grow along its limbs and tail. Six fiery tendrils grow from the back of its head like gills.
A scalamanx is an enormous fiery predator with a dim and cruel intelligence. They lair in volcanic caves and other sources of geothermal heat, and are found on the Plane of Fire despite their mortal natures. Although they have crushing jaws and thick powerful tails, the most fearsome weapon of a scalamanx is their fiery feelers. Each tendril of flame can move independently and lash like a whip, setting creatures and objects ablaze. A scalamanx is a stubborn combatant, and they frequently fight to the death once they have tasted the blood and char of a victim.
Scalamances have slow metabolisms despite their inner fires, and may only feed a few times a year. These meals are typically other creatures that stumble into their lairs, although during the driest parts of the year, a scalamanx may venture into the lowlands in order to hunt. These hunting forays can create wildfires that span for hundreds of acres, and many plants that need fire to propagate grow in areas patrolled by a scalamanx. At the first sign of precipitation, the scalamanx will retreat underground, as the touch of water burns them like acid, and any scalamanx caught in a rainstorm would surely die. Scalamances can understand a few words in Ignan, and respect powerful and intelligent creatures that can speak that tongue. Fire giants or efreeti use them as mounts, beasts of burden and guardians. Most scalamances are content to serve, but if they are mistreated may turn on their masters with lethal force.