One thing I have to differentiate in Cryptid Kings is which of the magical animals are wild and which are merely feral. There are some of these animals that are in some way or other, essentially roaming, untrained domesticated animals that you CAN make safe and CAN train if you do it right.
Grim are the big one, simply because one is a main character. Grim are large black dogs that fulfill quite a few “supernatural warning of misfortune” notes, and are drawn to conflict and humans in distress to try and alleviate said distress, typically by leading people away from the thing distressing them or, in the case of more aggressive Grim or more immediate danger, dispatching the source of said distress.
The big thing with Grim is that outside of some extra empathetic abilities (especially regarding people with anxiety, depression, or ptsd) they are no smarter than your average smart dog. They have some problem solving skills and can be trained to do a lot of things if you put in the time and effort, but you also have the problem that a bunch of Grim start as sort of “feral but attached to human/humans they like.”
Then you have all the downsides. Grim can walk through walls, and as one of my main characters, Ruth, points out, that means you cannot keep them out of anything. “Grim Dogproofing” involves putting it above their heads and hoping they don’t find a way to climb to it. Shuts doors and walls are nothing. This means you are going to be spending a fuckton of your life teaching them to Stay because otherwise they are going to follow you places they maybe shouldn’t be. Ruth had to be homeschooled for a year because her anxiety at school summoned her Grim, Sunny, and he was always in Defend mode which usually made things worse. It took a YEAR of concentrated work for him to Not Enter Her School and that put a lot of stress on her.
Grim also have pee that stinks worse than normal dog pee. If you do not hop on toilet training instantly you may be replacing a rug or some furniture. Grim eyes glow, so get used to your new faint nightlight. Grim do not react well to things that code as “solider” to them (unless the person coding “solider” is their person) and can become aggressive around uniformed armed forces, security, police, and heavily-armored paintballers. This may be because of Grim’s instinct to lead people away from conflict, so they assume a kitted-out “solider” means this is now a place of conflict and they need to get their people tfo. And since Grim can go intangible at will...never, ever fucking shoot them without the proper ammunition.
Because then they will get mad that you’re trying to hurt them, even as the bullet passes through harmlessly, and a mad Grim is fucking hard to call off unless they have a person who has trained them well. Grim can only be struck by certain quartz compounds, as certain quartzes are the only thing they cannot pass through. While Grim can swim, they seem confused by large bodies of water (oceans, seas, maybe the great lakes) and don’t quite know what to make of them.
Grim do seem to have a resistance to some common canine diseases, mostly tick-borne ones as ticks cannot get a firm enough hold on the frequently-intangible dogs. Fleas usually last longer, and mosquitoes and spiders can usually get a bite in. They also seem immune to Parvo, however most owners will still vaccinate them anyway, same with heartworm. Grim still need vaccines for kennel-cough, bordatella, and rabies, and it is advised to keep them on flea medication if keeping domestically.
Grim appear to live for around thirty years, and fade away rather than leaving bodies behind if they die of old age. The actual complete lifecycle of Grim is still a little sketchy, as most are taken from firsthand accounts of Grim owners due to the difficulty of keeping them in one place for study otherwise. Curse those ‘walk through walls’ dogs wallwalking abilities!