Made from a mysterious black metal that retains a form of sentience (?)
Ball and chain
Strange black metal, looks and feels like iron. Far stronger and far heavier than expected.
Granted some form of sentience, acts like a dog. Sharp teeth, dopey demeanor, licks faces, growls and barks. Genderless (no biological anything, but still eats food. Really likes iron shavings and metal materials). Drinks water, doesn't rust. Tongue feels likes a dog tongue, slobbers and drools like a dog.
Heatproof and conductive to electricity
Literally showed up one day outside of the forge, sniffing and poking around for something to eat. Clearly magical, Yama decided to take it in and see what it was like. There was no collar, no name, and a pink bow on the side. He didn't want to immediately label it a female, as it was just a sentient ball and chain. Possibly a magical experiment gone awry?
As soon as it was let in, it got to gnawing on the anvil. It's teeth were sharp, but not enough to bite off a chunk.
"Hey! No! Bad... dog?"
It then stopped gnawing and bounced towards him, thud, thud, thud, panting like a dog.
So, it responded to commands, that was good, but a low rumble emanating from the part closest to the ground nearly shook the floor. It gave a few sniffs and then hurried towards the grindstone where a pile of shavings was from a broadsword build. It began snarfing down the shavings hungrily, better that than his tools, and licked its... chops? There were no jowls, just a mouth with sharp metal teeth that interlocked perfectly.
Something brushed against his leg, and he looked down to find a thick chain that was connected to the thing's backside. He picked it up, and the metal monster went silent. Its button-like eyes stilled and the maw closed tight. He dropped the chain, and life entered the creature once more.
He'd watch over the creature for now, maybe take it to a fight or two, but it would need a name. For two days, Yama wracked his brain for something was wasn't something based on "chomp."
He wasn't good with magic outside of Bending, but Yama felt as though this was something that was either enchanted or it was an alchemical experiment gone awry. Well, the poor thing didn't want to leave now, and who could say no to that adorable face? He could hug it as hard as he could and not worry about squishing it!
After a week or so, Yama decided that if this thing was going to roll around and eat scrap metal, he may as well put it to work, and began to train it for combat. After training it not to jump on people, as his mother was nearly crushed and his father had to lift it off of her when they came to visit.
Because it is a massive ball and chain, Yama swings it around as such. Using Ferrenom's massive reach and sharp teeth, it can be used to do some serious damage and even grab onto things. Nothing is safe from its bite, as it can even crunch through stone. At one point, Yama hooked the chain onto a ceiling and used it to drop a boulder as a trap while he was fighting.
Of course, when Yama goes out alone, he leaves it to its own devices. It usually sleeps all day and eats from a bowl of scrap that Yama has saved at all times. It also drinks from the stream flowing under the Forge, so water isn't an issue.