"You cannot fear a thunderstorm if you ARE the thunder!"
The name for a Jolteon in Japanese is "サンダース", pronounced "Thundersu".
"Dear" spelled phonetically in Katakana is "ディア". These words are combined to create an English pun, サンディアス "Thundear-su".
Zapdos' name in Japanese is "サンダー", just a one character difference between it and Jolteon. This is maintained in English by having its name start with "Zapd".
While its name is spelled "Zapdear", outloud Red pronounces the "dear" like "dier", like in the word "terrier". This maintains plausible deniability.
Eevee are incredibly rare Pokemon, rarely ever seen in the wild. Zapdear was born in the house of someone who specifically breeds and sells Eevees. It was taken care of primarily by its mother Eevee and only socialized with its siblings.
As a young pup, Zapdear was given to a trainer. This trainer wasn't very interested in the Eevee however, and mostly kept him in his pokeball.
One day, his trainer released him from his pokeball on the roof of a department store. Then, the trainer left. Zapdear was stranded on the roof all alone, with no way to get down. He could not operate an elevator, and as the building is several stories tall he could not jump down either.
Though it could bark, no one heard it. As it wasn't trained either, it didn't know any attacking moves that could help it either. The Eevee was all alone with nowhere to go.
While it was up there, a thunderstorm hit. There was nowhere he could shelter himself from the rain, and no one came to rescue it. It was all alone and all it could do was watch people from below running away with their umbrellas, and watch the flickering lightning of the clouds.
Red (then "Dawn") traveled to Celadon during his journey to collect the gym badges. Well in the city, he decided to check out the department store for any useful items he could buy for his Pokemon.
On a whim fueled by childlike curiosity, having never been on the rooftop of a tall building before, he goes up there, and spots the abandoned Eevee.
After deciphering what had happened, he offers to help the Eevee get off the roof. Eagerly the EV clings to him and he carefully carries it down to the elevator and outdoors.
Once outside, Red isn't sure if he knows anywhere safe nearby he can drop the Eevee off to. He offers Eevee that he can take care of him, and once they find somewhere safe Eevee can decide to leave or stay part of his team. Eevee, not used to having very much choice in his life, accepts this.
Because Red didn't have a Pokeball for this Pokemon, the Eevee would follow him by sitting on his shoulder or walking beside him.
There it got the most experience that it ever did and its entire life, being able to witness cities, people, and trainers. It was there to witness Red fight for his gym badge, and it wanted to be a great Pokemon like his too. After that, Red was able to have the Eevee officially transferred to him.
Back then, it was just called "Dear".
Even once Red had a Pokeball for Dear, it would still often stay outside of it and follow him around. He wasn't especially clingy, but still couldn't fathom the reason why someone wouldn't want him. Dear knew he was a special and rare Pokemon, but did not know the reason why. All he knew is that he was special and it made no sense why his previous trainer would abandon him.
In a random trainer battle, Red battled against a trainer who had an electric type. Red had no electric types on his team, and hadn't fought one yet, so that battle was the first time that Evie had ever witnessed a Pokemon using those attacks. It was incomplete awe.
Red noticed, and asked if he wanted to be a Jolteon. As a pup, Dear he was never educated into what he can evolve into, so didn't know what that was. Red took the task upon himself to teach him.
Now aware of the biggest decision that it would ever make its life, Dear thought often of what to do.
Dear could evolve into a Flareon, grow passionate like fire, and learn the art of the flame from Sunny the Arcanine. He could always stay warm and cozy.
Dear could evolve into a Vaporeon, learn the secrets of the sea from Bubbles the Horsea. The rain would never bother it again.
Dear could evolve into a Leafeon, learn to listen with the plants and grow its own gardens with Venusaur. Then he could play in the dirt as much as it wanted.
Dear could become a Glaceon, learn to fight fearlessly and control the cold with Puddles the Lapras. Then it would command the cold rather than fear it.
Dear could evolve into Espeon, learn a whole language that it had never known before with Pearl the Butterfree. With those powers, he may even be able to surpass her.
Dear could evolve into Umbreon, a type which Red's team lacked. He could become one with darkness, and never fear the unknown.
Dear could become a Jolteon...
The cloudy skies that covered the sunlight, brought forth chill, rained upon all that was under, which crackled with lightning that taunted the Eevee on his worst day could be under his control.
Dear would be special, the only one on Red's team capable of commanding such a force.
It's mind was made.
Zapdear wears the evolution stone Red bought for it as a necklace.
Using the electricity built up within its body, it can stun any opponent. If an attack requires an especially large amount of electricity. he can bring down thunder from the skies, sometimes causing temporary storms.
He can buzz and click and zap with any of the noises electricity can. If it wants to be heard from a distance, he imitates large thunder cracks which some people have mistaken for gunshots.
Zapdear was the leader for the other pokemon. He would keep the group organized, give them tasks, and make sure no one got left behind. With his thunderous bark, his was able to communicate from any distance and signal where he was even in blizzards.
Using his sharp claws and quick feet, he would often dig holes to open entrances to caves, make holes in the ground to huddle in for warmth, and help Leaf the Venusaur with the garden.
While on the mountain, Zapdear developed a thicker coat and white patches on his fur. This helped with staying warm and with camouflage to stay hidden from wild pokemon and capture prey if needed.
Even once leaving the mountain and shedding its thicker coat, Zapdear's white patches of fur never went away. He lives with Red and Blue in their Pokemon Gym, far too powerful to use against challenging trainers, so instead spends most of his time running around and playing with the other Pokemon.
Zapdear is very proud of his power, and lives confidently as a king of the sky.