Jagger: The Dog from Elsewhere — Alexander Key

A magical dog falls through a portal and finds himself in Alabama

Jagger is a protector of the hunted -- a great white dog who stands between the people of his village and the evils that lurk outside of it. When an earthquake splits open the ground beneath his feet, Jagger travels through space to a mysterious forest, where a deer is running for her life. To save her, he fights off a pack of vicious hounds and gets shot in the leg by a pair of hunters. As he lies bleeding in the woods, running out of hope, he cries out with his mind -- and a young girl's voice answers.
 
Nan Thornberry has always had the ability to talk to animals, but she has never met one like Jagger. She and her older brother rescue the dog and bandage his wound. In return, he will serve as their protector, guarding them against the greatest evil he has ever had to face.