Mircea is Charlie Weasley's dog.
- Mircea is a wild magic lycanthropic hybrid
- He is strong and fast, never gets sick, and heals quickly
- He'll live to be about 20 and is only halfway through his expected lifespan instead of old for a dog of his size
- He requires a meat diet
- His aggression levels and energy/restlessness levels are tied to the moon, and are very high at the time when werewolves shift
- He's extremely protective of Charlie and other residents of the Burrow
- He's an alpha who has accepted his lycanthropy
- He smells like werewolf and dog to other weres
- He can detect other weres by scent
- He likes other weres
Charlie is generally oblivious to the supernatural characteristics of his dog. Mircea is just a great dog, that's all.
During the war, Charlie's in a battle with a werewolf which attacks his two dogs. Being Charlie, the insane animal lover he is and the super amazing dog daddy I'd assume he is as well, he unwittingly causes a reaction from wild magic so the dogs don't die but become, essentially, dog/lycanthrope hybrids. Instead of them becoming something that changes physically with the moon like human-based lycanthropes they have more an internal change that goes on.
Working off the same principle as with humans you have the factor of accepting or not accepting the change when it happens. The dogs, one could logically say, weren't exactly going to be accepting of the disease's introduction to their system and tried to reject it. This would cause them to have essentially die of old age before they should have because of the internal battle of their system versus this disease. Meanwhile, however, Charlie's bred these dogs. The disease itself kills most of the litter, just like it would in human babies when someone is or gets pregnant by a lycanthrope. The only one who survives is Mircea. Because he's a puppy he's born with this lycanthropy hybrid in his DNA already, he doesn't have the choice of acceptance or rejection, it's just what he is and so he ends up on the acceptance end.
What you end up with in Mircea isn't obvious because you've essentially integrated two different canine species, wolf and carpathian sheepdog, instead of two completely unrelated species, homosapien and wolf. He doesn't have to change form or battle a foreign presence inside of him because the sheepdog in him would recognise wolf. It's dog, it's just wild in a lot of ways. What you end up with is small stuff. His perceptions and senses would match that of a wolf. He'd live much longer than he should (lycanthropes live longer than wizards already) so he's probably only about halfway through his life instead of nearing the end of it like he should. Mircea can easily live to be 20, something nigh unheard of in large dogs and damned difficult to achieve in small ones as well. Wolves are carnivorous so Mircea likely needs to eat meat, not dog food. Dog food has a lot of filler like rice and vegetables but on such a diet Mircea's health would suffer. If he's fed meat, preferably red meat, he'd be a happy, happy dog. He also likely needs to eat more than a normal dog would to maintain his metabolism. He wouldn't get sick because the lycanthropy in him would fight off anything else that may attack him, and he'd heal injuries pretty well. He'd be faster and stronger than a normal dog of his breed, closer to that of a wolf but actually pushing what most people would consider reasonable for a dog of his size and stature. Mircea wouldn't be a very fat dog, he'd be lean from an increased metabolism thanks to the lycanthropy.
He would smell like a combination of werewolf and dog, unlike someone like Finn who would smell just of werewolf all the time to someone like Fred who could tell the difference. During full moon as well as the two days previous and following (which are the five days when normal lycanthropes shift) he's likely got more energy, is maybe a little restless and wants to go out and run a lot, preferably he'd want to hunt. There wouldn't be any physical change, though. He'd be damn protective of Charlie and likely any one else who lives in the Burrow, seeing them as his pack. If threatened he'd take on a role of an alpha, being born a lycanthrope he'd naturally be an alpha just to have survived to be born in the first place.
He'd like lycanthropes, though. If Charlie ever realised what Mircea was it'd be a really easy way for him to know who was or was not a lycanthrope. Dogs like people anyway, they're social creatures, and they like other dogs in a way that cats don't share when it comes to other cats, you know? So a lycanthrope, to Mircea, would be like another dog in the park. He'd sniff them, feel them out, and want to play with them, befriend them. To him a human lycanthrope would be just like him and he'd recognise that on an instinctual level to the point where he might try to get Charlie to befriend lycanthropes so Mircea could have them around, just like how dogs try to get their owners to go to dog parks so they can be around other dogs.