I believe they are souls, both animals and human, unfairly killed that wish to help the living. If they are animals, they are granted speech, language, and understanding if they wish. They are assigned, or chose, a role in which to protect you and they are with you from birth. They have the ability to take many forms.
I also believe that spirit guides can be those who unfairly kill those souls if the soul they killed did not want to do it. It is seen as a form of redemption, of some sort, before they can reincarnate.