This got a pretty good reaction on deviantart.
Note: This is meant to be read imagining the characters as gijinka/humanoid Pokemon.
Warning: May cause crying.

The Magikarp and the Feebas

Once upon a time, there was a little boy.
He was a Magikarp, so no one really liked him.
No one cared about Magikarp, everyone thought they were useless.
This boy was no different.
He was alone all the time, swinging on the swings at his school all by himself.
Magikarp was used to it, but it still hurt him.

Then one day, a girl joined him on those swings.
She was a Feebas.
Feebas and Magikarp had certain things in common.
They were both fish Pokemon,
and they were both thought to be useless.
Magikarp always thought Feebas got worse treatment.
Everyone thought they were ugly.
This girl seemed to like him,
and he saw nothing ugly about her.
"I don't care what anyone says", he said to her one day.
"You are beautiful."

Magikarp and Feebas grew up together.
They were best friends.
They protected each other, and were always there for each other.
Magikarp knew that he felt something for Feebas;
he just wasn't sure what.
Was it love?
He trusted her.
She was his only friend.
He told her she was beautiful every day,
because in his eyes,
she was.
He couldn't understand why others couldn't see her beauty.

One day, Feebas evolved.
She was a beautiful Milotic now.
Inside and out, she was so beautiful.
Magikarp was very happy for his friend.

But then, something happened.
Other Pokemon looked at the newly evolved Milotic.
They started treating her differently.
They were nice to her, they treated her great.
While Magikarp was happy his friend was finally getting some respect,
he noticed something.
Milotic had started to drift away from him.
She would spend more time with those other Pokemon,
and she would hardly have any time left for him.
He was happy that she was making new friends,
but he missed her dearly.

They talked less and less every day.
Magikarp knew that his friend didn't mean to drift away,
but it still hurt.
He tried to remember all the good times they had together;
just the two of them, but it only made him hurt more.

Eventually, they stopped talking completely.
Magikarp never saw Milotic anymore.
He knew she didn't mean it.
She was always surrounded by her new friends.
They were stronger, bigger Pokemon.
Magikarp knew he could never stand up to them.
He was just a useless little Magikarp.
He could barely walk without tripping over his own feet.

His heart hurt.
He felt so alone, more alone than before he even met Milotic.
Those strong Pokemon who were her new friends,
they never let him talk to her.
All Magikarp could do was look at her from a distance,
hoping that she would make eye contact with him,
and come rushing to his side.
She would have done that before.
No matter what though, he didn't blame her.
It wasn't her fault.
He knew she would never hurt him on purpose.

Magikarp decided one day that he wanted to get stronger.
If he got stronger, maybe he could be her friend again.
So Magikarp trained and trained.
He trained every day.
And eventually, his training paid off.
He had evolved into a Gyarados.
He was so happy when he evolved.
He could finally be friends with Milotic again.

Gyarados rushed back to his hometown,
where he knew Milotic would be.
The other Pokemon in the town hardly recognized him.
He looked for Milotic everywhere, checking all over the town.

But he was too late.
He learned that Milotic was to be married.
She was going to marry Salamence.
Gyarados remembered that Pokemon from before he left to train.
Salamence was the first one who noticed Milotic's new beauty.
He was the one who stole her away.

Gyarados saw them the next day.
They were walking together, and he hid before they saw him.
He looked at Milotic.
She was as beautiful as ever.
She looked so happy.
Gyarados sighed and turned away.
Milotic was happy with Salamence.
If she was happy, then it was okay.
He didn't care.
As long as she was happy.
He left his hometown for the second time.
He didn't stick around for the wedding.
He just couldn't.

As he traveled, he met many other Pokemon.
If they wanted to know, he would tell them his story.
They would tell him they were sorry for him,
and he pretended that he didn't really care.
But he did.
He cared so much.
He loved Milotic.
He really did.

Gyarados would love Milotic until the day he died.
He knew he would.
He continued traveling, looking for a new place to call his home.
Eventually he settled in a cave hidden within a waterfall.
To distract himself from the pain of his bleeding heart,
he trained.
It was the only other thing he knew how to do.
Gyarados trained, and he trained.
He got stronger and stronger as the days went by.
Not once did he ever forget Milotic,
and he hoped that she was happy,
wherever she was.

If you were ever to travel,
you might come across other Pokemon.
Some of those Pokemon might tell you a story
they had heard from a traveler themselves.
They might tell you a story
about a lovesick Gyarados,
who hid himself away in a waterfall far away,
building up walls around his shattered heart.

If you were ever to travel,
you might make it to the far mountains.
If you ever made it to the far mountains,
you might find a cave hidden in a waterfall.
If you ever find that cave, hidden in a wall of water,
you might find a Gyarados.

A Gyarados who, to this day, is said to be waiting for someone strong enough to break the walls he's built around his heart.

I feel I should explain the whole process of evolution that I'm following. I've always liked the idea that Feebas evolves when they are told that they are beautiful, and both they and whoever said it believes it with all their heart. As for Gyarados, as Gyarados is more for power and Milotic for beauty, it is the opposite. He evolved because he wanted her back, and it drove him to keep training.