I remember previous reincarnations and I can tell you that several negotiations go on before actually being born again. I know that much, but I am not sure about at what point in time it is decided who will guide you. I'm not yet sure how all that works. To my knowledge, they are [suppose to be] with you since the day you're born. That is why I have reason to believe that previous negotiations possibly contribute to the picking of your guide. I'm not absolutely positive about this, though, so don't take my word for it.
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.