Soot on any part of a fire-type’s body is an indicator that its body temperature is dropping. That, in turn, is typically caused by exhaustion, stress, or a mild cold. My advice would be to let your fennekin rest and perhaps take her to the pokémon center if she seems lethargic, her nose feels dry, or she’s having trouble eating.
Best of luck, anonymous!
It’s possible to separate them as soon as fights break out. As in, physically separate them and tell them in a firm voice to stop fighting. This may seem like a basic thing, but if you show both your friend’s fennekin and your vulpix that you’re neither afraid of them nor afraid of breaking them up and giving them a stern talking-to, then they’ll be more likely to listen to you and behave (in your presence, at least).
It’s also worth it to note that vulpix and fennekin are both fox-like fire-types that evolve into pokémon with mystical abilities and that fennekin, as starter pokémon, are known for being more docile and friendly than the considerably rarer, prouder, more capricious vulpix. That is to say, it’s important to recognize that your vulpix may feel threatened by your friend’s fennekin, so it’s important to give your vulpix its own space and amount of affection. Reassure her that you don’t plan on adding your friend’s fennekin to your team and that she doesn’t have to interact with that fennekin if she doesn’t want to—but that you need her to be on her best behavior if she does.
As for your friend’s fennekin, they’re likely suffering from a simple case of homesickness, which happens when a pokémon isn’t in the care of the human they’ve bonded with. Because of that homesickness, though, your friend’s fennekin may be feeling on edge, which isn’t helped by the animosity your vulpix is feeling due to the aforementioned worry about being replaced. So it’s important to reassure fennekin that your home is its home for the time being and that you’re willing to do whatever it takes to make it feel safe and comfortable. You can do this by giving this fennekin its own space and amount of affection while keeping it away from your vulpix.
If you’re still struggling to keep their tempers in check, it may be a good idea to ensure your vulpix remains in a separate room from your friend’s fennekin. Use their poké balls if you need to, and set up a time when your friend’s fennekin is out and taken care of while your vulpix is in her ball and vice-versa. Be sure to keep their toys and possessions separate and out of reach during each shift.
I know this sounds as if I’m saying it’s impossible to get them to socialize. It’s not. In fact, after rotating them in this manner, it’s possible that they’ll understand why they’re being rotated and attempt to make amends with one another. However, the main point is this is more of an issue you should work out with your friend present, rather than with them absent, as it will involve training on their part as well. You will need to work with your friend—the fennekin’s true trainer, with whom it’s bonded—to bridge the gap between your vulpix and their fennekin and to get the hostility between them to calm. You can only do so much via training your vulpix.
Best of luck in the meantime.
Before anything else, are you certain that this fennekin always acts like this, even when in the company of its proper trainer? Sometimes, stress can cause pokémon to revert to behaviors associated with younger, untrained pokémon, and the kinds of behaviors you’re describing can also (rarely) occur in pokémon that have been traded. This is because they’re under the impression that they’ve been abandoned by their former trainers, so the first step is actually to reassure them that they haven’t been abandoned at all. It’s just that in your case, whereas traded pokémon need to be reassured that their new trainers love them, you have to convince this fennekin that its old (or, well, current) trainer loves it. This is why she hasn’t recalled fennekin and deposited it in the storage system for the few weeks she’s gone, after all.
So although it’s a bit too late to ask if your friend has a routine with fennekin, it’s worth it to establish one now. Give this fennekin the same amount of attention, affection, and training you give your other pokémon. Use this fennekin in battle to help establish a bond and a level of mutual respect. Do everything you can to treat this fennekin as one of your own pokémon, and it will begin to acclimate to your home.
If this is just typical behavior for your friend’s fennekin or if you need to give her guidance on behavior in yoru home, meanwhile, use positive reinforcement and begin with the simplest thing to correct: housebreaking. Lay down newspaper and whenever you see this fennekin relieving itself anywhere but the newspaper, pick it up and put it on the patch you’ve covered. Any time it relieves itself on the newspaper, reward it with a treat. Eventually, it will begin to associate relieving itself where you tell it to with a reward, which in turn will make the rest of its retraining far, far easier. Use the same tactics for each of the other behaviors. When you see fennekin clawing your furniture, push it off and give it a toy. If it plays with its toys instead, give it a treat for doing so. Place your pokémon’s food bowls far away from this fennekin’s, and any time you see it eating from your pokémon’s dishes, pick it up and carry it to its bowl. If it eats from its bowl (and only its bowl), give it a toy. Reinforce these lessons with words. A simple “no, Fennekin” in response to bad behavior and “good Fennekin” in response to good behavior is enough.
Of course, all of these lessons may be moot if your friend doesn’t actually train her pokémon, but at the very least, this fennekin will understand proper behavior in other people’s homes by the end of this training session.
As for what you should tell your friend, because of the first paragraph in this response (and the uncertainty of whether or not this is normal for her fennekin), it’s extremely unwise to accuse your friend of failing to train her fennekin. However, it’s also vital to be open with your friend, in case this is indeed an issue of either bad training or separation anxiety. Tell your friend that you’ve noticed some strange behavior in her fennekin at first and describe the behaviors you’ve encountered. Also tell her what you’ve done to help her fennekin adjust to your home but never accuse her of being the source of her fennekin’s problems. If you phrase it politely enough, you may be able to convince your friend to look into either her fennekin’s mental health state or reinforcing what you’ve taught it.
In short, yes, it’s perfectly fine (for the sake of both your sanity and your home) to train your friend’s fennekin. It may even be vital to its mental health. It’s just that when it comes time to give it back, be as gentle as possible to both the trainer and the pokémon.
Fennekin
The Fox Pokémon
Type: Fire
Official Registration #: 653
Entry: One of three pokémon traditionally offered to new trainers at the beginning of their journeys in Kalos. Fennekin are popular starters for their adorable appearances and behaviors. They are frequently found trailing after larger pokémon or their trainers while chewing cutely on twigs they find along Kalosean forest paths. These twigs fuel fennekin’s internal fires, so in order to combat this, they shake their large ears cutely in order to expel hot air. Considering the number of cute traits and the species’ identity as a fire fox, it should be worth it to note that the vulpix line, a fellow fire fox evolutionary tree noted for their vain dispositions, do not get along with fennekin and have been known to engage in vicious physical assaults on fennekn out of jealousy, despite the fact that the two lines are actually distantly related. Therefore, it is ill-advised to leave a fennekin in the same room as a vulpix. Unless, of course, you happen to be the sort of trainer who is somehow entertained by reality shows such as Undella Shore and Real Housewives of Orre … at which point the author respectfully asks you to relinquish your trainer’s license at the nearest pokémon center.
Braixen
The Fox Pokémon
Type: Fire
Official Registration #: 654
Entry: The evolved form of fennekin, by battle experience. This bipedal fox-like pokémon keeps a twig in its bushy tail when not engaged in battle or in the vicinity of fellow members of the fennekin line. In either condition, the braixen unsheathes this twig, the tip of which—as a result of friction against its stiff, flint-like hairs—catches on fire as soon as it’s fully drawn. Once lit, braixen may use this light to communicate with its fellow pokémon by waving its twig in elaborate patterns before it. A visual representation of multiple braixen communicating by twig fire with one another may be seen here.
Delphox
The Fox Pokémon
Type: Fire/Psychic
Official Registration #: 655
Entry: The evolved form of braixen, by battle experience. A five-foot-tall fox gifted with powerful flame and psychic abilities, delphox’s signature attack involves combining its elements to create a vortex of fire that reaches temperatures of up to 5400 degrees Fahrenheit. In theory, this attack draws in its opponents and inflicts a devastating blow of the fire element. In practice, given the fact that a vortex of hot air creates a powerful vacuum, this attack draws in literally anything around it save for itself and inflicts a devastating amount of property damage.
The Pokédex According to Bill 