Gyarados (the Atrocious Pokémon) is an uncommon Water/Flying-type Pokémon found among bodies of water within and around Asia, the Kalos region and some special preserves. Violent and ruthless to the extreme on even a good day, Gyarados are monstrous beasts that more than make up for the inadequacies of their pre-evolved form with sheer power and devastating versatility on the battlefield. They might not end up learning as many moves as some species of Pokémon, but for Gyarados that is rarely an issue, as all it takes is a single Hyper Beam or Dragon Rage attack from one of these beasts to turn a simple village into a blazing inferno of destruction!
While it is believed that ancient Magikarp likely resembled their evolved form a great deal, Gyarados have pretty much stayed the same throughout their history on the planet and are effectively one of the most fearsome fish to ever be seen in battle. When Magikarp evolve, a massive chemical reaction within their brains permanently damages their adrenal gland and makes it almost impossible for these beasts to turn it off; in short, the slightest bit of anger can cause their bloodstream to be overfilled with adrenaline, leading them to act violently and go on a rampage that can last for days, weeks and even a whole month in some cases. Unlike Magikarp, Gyarados are extremely powerful beasts that are heavily fortified in terms of offense and defense and are surprisingly quick for their large size, but what really makes them a threat are the attacks they manage to learn after evolution. Aside from gaining access to the powerful Bite and Thrash attacks immediately after evolving, Gyarados gain access to a decent variety of Normal, Water, Dark, Dragon and Ice-type moves as they develop and eventually gain the capacity to utilize the powerful Hyper Beam attack, though they are unable to do much with it in battle as a result of their low base Special Attack stat. Even so, Gyarados are extremely difficult to tame and are virtually unmatched in the water as a predator, as their sharp fangs and monstrous jaw strength allow them to smash through tough shell surprisingly quickly. It is lastly noted that, strangely enough, Gyarados are classified as both Water and Flying-types because they actually have the capacity to fly for short distances above the water and their cells are more conducive to electric shock than those of similar life forms. While they don't learn any Flying-type moves naturally, this can at the very least give Gyarados a surprising edge over ground-based Pokémon in the wild, a trait that proves especially useful because Gyarados can survive in both fresh and saltwater and thus may end up switching habitats during mating season.
Since Gyarados are effectively in a permanent bad mood, it is very rare for them to be captured let alone battled in the wild and the mere thought of doing so would probably be the worst course of action that could ever be taken. This is particularly evident in the fact that Gyarados do not favor humans much for the violence they cause and the pain they inflict on themselves and others. History has shown this to be especially true during times of war within and around the Japan area, as there are many tales involving Gyarados becoming intensely aggravated by the smell of burning objects and the sound of weapons clashing in battle, even to the point where they intervene and cause some massive devastation of their own. The Hyper Beam attack of these beasts may not be very powerful, but because of how these creatures generate the energy needed for the move, their Hyper Beam acts like a powerful beam of thermal energy capable of setting ablaze anything and everything in their path, leaving little else other than charred rubble in their wake. Worse yet, some Gyarados have learned how to turn their Dragon Rage attack into a devastating weapon by using the sheer power of the attack to generate massive water cyclones capable of sinking an entire luxury ship. Needless to say, this has made Gyarados both feared and respected creatures in their homeland and can more then make them a dangerous tool in battle, though the chances of getting one under complete control is still difficult if not impossible in many cases.
It is lastly noted that Gyarados is one of only 46 species of Pokémon known to be capable of undergoing Mega Evolution. While most Mega Evolutions retain the same type assignment as their normal form in battle, in the case of Gyarados, the development of a radical new anatomy completely changes what they are capable of doing in battle. When Gyarados Mega Evolve while clutching a Gyaradosite, their general length does not change but their mass increases substantially as a result of massive development in the body muscles located near their head and the front half of the body, making them much bulkier than normal. In addition to this increase in muscle mass, these beasts mutate and lose the extra conductivity normally found in their cells and grow a substantial energy conduit in the front half of their body that can charge up and energize their form with negative energy, turning them from Water/Flying-types to Water/Dark-types. This newfound type assignment comes with a massive increase in aggression, so much so that even the best of trainers can find it nearly impossible to fully control a Mega Gyarados if it becomes enraged. In terms of stats, this radical change in anatomy to Mega Gyarados slightly increases their base Special Attack stat and considerably increases their base Defense, Special Defense and Attack stats; in turn, they gain the highest base Attack stat out of all Water-type Pokémon, the highest base stat total out of all non-legendary Water-type Pokémon and the highest base Special Defense stat out of all Dark-type Pokémon (tied with Umbreon). The increased aggression observed in the species also has the effect of making it attack others so fiercely and so vigorously that it has a habit of striking its opponents without much care for their Abilities, breaking through their innate defenses to the point where they are effectively rendered possessors of the Mold Breaker Ability. Altogether, this makes Mega Gyarados a monstrous sight to behold in any battle and most certainly not an opponent that any trainer will want to face without a serious type advantage for protection, and even that might not be enough.
Though it might be a pain for some to have to deal with Magikarp until they manage to evolve, Gyarados are titanic fish more than worth waiting for in practice. With the highest base stat total out of all non-legendary Water-type Pokémon (tied with Milotic and Kingdra) and a devastating selection of moves sure to make the opposition cringe, it goes without saying that these beasts are the among the crème of the crop when it comes to sheer versatility and power in the water. Granted, they might be easily downed by a single strong Electric-type attack, but as long as the opposition isn't incredibly fast, it shouldn't take too long to finish them off in kind; just make sure that you can keep your Gyarados from becoming too aggravated, or you might end up paying for more property damage than you can budget!