Bumper

Name: Bumper
Nickname: [What do you call your pet? This is optional.]
Age: 15
Species: Horse
Appearance: He has brown fur and white fur between his eyes, with a flowing mane.

Spoiler:

Personality: He is very particular about who rides him. Yami uses a special code phrase to make sure he can trust her. After using the phrase once, he usually lets people ride him after that without having to repeat the phrase. However, if somebody gets on him without using the key phrase, he tries as hard as he can to throw the person off.

Bumper also protects his friends. Yami has only been with him for about two years, but he trusts her, and she is also able to trust him if ever there is immediate danger. He tries to protect Yami, even at the cost of his own health.
Background: It was just after Yami left her village. She was tracking the people who attacked them, trying to find where they went, when she heard a commotion in some nearby trees. There, she found Bumper, surrounded by a group of men dressed the same way as the men who attacked her village. As soon as they saw her, one of them recognized her, and called out, "That's the girl! She's the one who fights like a demon!" he cried. The others just laughed, but when Yami drew her sword, a few shuffled their feet uncertainly.

"What's going on here?" she asked, noticing the distress of the horse in the middle of the circle. "Uh, the horse, he belongs to us. But he doesn't listen to a thing we say. He doesn't let anybody ride him! So, we uh...we were gonna kill him." The first man explained, a look of shame on his face. "There you go again, treating life like it's nothing. Alright, I will let you live, but on one condition. You have to leave me the horse." she said.

"Fine, take him. We don't need him anyway." and with that, the group departed. The horse turned to face Yami, a puzzled look on its face. "Don't worry, boy, i'll take care of you." she said. The horse whinnied happily, and something fell out of his saddle. A small piece of paper. Yami picked it up curiously. So, you found the paper. Congratulations. If you are reading this, that means you either rescued Bumper or kidnapped him. But, like it or not, Bumper has taken a liking to you, and now you are his master.

To ride bumper, simply say the words, "Don't break me!" and you are then free to ride him from then on. If this phrase isn't spoken, he WILL throw you off. I trust you will take good care of him.  and the note ended there, with no signature. Yami smiled and looked up at Bumper. "Don't break me!" she said. He just sat there, and Yami climbed on him, and he whinnied happily again. Yami smiled for the first time since the disaster, and took bumper riding off into the distance.