Well, as noted in an earlier ask, you’ll want to be cautious of introducing anything that can breed with gyarados into a gyarados pond. This means any female fish-like pokémon may be a bad idea, as presenting a gyarados with one as a companion may trigger his more primal instincts. Male specimens, however, are fine, and there are quite a few stories of lifelong friendships between milotic and gyarados. (It’s likely because both of their preevolved stages can commiserate over being formerly the weakest fish in a pond, so to speak.)

The other sorts of pokémon that would be bad ideas to keep in your gyarados’s pond would, of course, be chinchou and lanturn, as they sometimes discharge electricity into their habitats. As you likely know, gyarados in their natural states possess a potent weakness to electricity, so being that close to either chinchou or lanturn may be dangerous to your gyarados’s health. Try to keep him away from anything that knows electrical attacks as well. Starmie that you obtain from a trainer or breeder, for example, may know Thunderbolt, and specially bred remoraid and octillery may know Thunder Wave.

All other pokémon that live in the water are excellent choices, but gyarados prefers anything it can swim with. Part of this is because gyarados tend to be highly competitive, but some part of it is left over from its time as a magikarp. (Magikarp congregate in schools and shoals to protect themselves, after all.) If you can get a pokémon that’s native to the same place your gyarados is from, even better. If you received your gyarados from a breeder, obtaining a local pokémon from the same sort of environment magikarp generally live (such as marill, wooper, psyduck, tentacool, and so forth) will do as well. This is largely because, as these pokémon are native to gyarados habitats, they’re more likely to be used to a gyarados, or they’ll at least adapt well to your gyarados’s presence.

In short, most water-types are perfectly safe to raise with your gyarados, especially given the fact that you’ve said he is well-trained and understands the difference between food and everything else. But the closer you can get to pokémon that would normally live with it, the better. If not, simply stay away from anything it would want to mate with or that its entire species is weak to.

First and foremost, congratulations on your impending evolution, anonymous.

Second, although it’s not unusual for a pokémon’s personality to undergo a shift during evolution, remember that above everything else, your magikarp will still be your magikarp inside. So long as you’ve given it the best care as possible and so long as you make it clear you’ll continue to do so after evolution, your gyarados will have no reason to hurt you.

In fact, in many cases, although wild gyarados are ferocious and violent, tame gyarados, particularly those you’ve raised as magikarp, only act out if you make it known that you’re afraid of them or if you make it clear that you’re about to hurt their owners. Otherwise, they tend to be rather docile and incredibly loyal. This is true for practically any “ferocious” pokémon. The quicker you establish the idea that you’re both willing and capable to meet their needs as a caretaker and a trainer, the quicker they are at bonding with you. If you find it difficult to remind yourself of this, take a moment to ground and center yourself (meditate, in other words) in order to achieve a calm mental state before approaching your gyarados.

As for care, the most important thing to remember is gyarados are both carnivorous and far, far bigger than magikarp. You’ll need a large body of water (a lake or olympic-sized swimming pool) to give it plenty of room to exercise. Feed it at least sixty pounds of raw meat and fat at least every twelve days. (Tamed gyarados can have their meals broken down into five pounds once a day daily, but some gyarados prefer larger chunks. This should also be reassurance that if you’re unable to feed your gyarados on a particular day, it can survive. Doubly so if it’s an outdoor gyarados, as they tend to supplement their own meals with birds that happen to fly over their habitats.)

Gyarados also tend to roar and snap at random, but these aren’t normally displays of aggression towards their trainers. Rather, it’s often a gyarados’s way of playing—or hunting, if it happens to spot something flying overhead. These acts are perfectly harmless, so it’s recommended that you don’t prevent your gyarados from engaging in them. However, if you absolutely must, do so in a reassuring tone but explain things clearly to it. Contrary to popular belief, gyarados are highly intelligent pokémon and understand the meaning of the word “no” if told by a trainer they’re already loyal to.

Lastly, never keep a gyarados of the opposite sex in the same habitat as this one. Gyarados become difficult to reason with when mating, and their rituals are highly aggressive, loud, and violent.

Good luck, anonymous.