Kramer is Jake's horse.
Jake and Mallory rush over to Heartland to ask for Amy's help rounding up Kramer. He developed a habit of jumping the fence and Jake's parents want Amy to help or they will re-home him. Amy discovers he's jumping because he loves to so Jake lets Mallory ride him to learn to jump.
When Jake starts dating Chelsea, he goes to heartland to ask for Kramer back so Chelsea can learn to jump him.