Does anyone know anything about EVP's?

I have this realy messed up EVP. Its my boyfriend as a baby, his sister, and their mom and grandma. They are all playing and talking, then the tape cuts out and theres all there eerie sound. There is an animal (more like a beast) you can tell its huge, it growls, opens creeky doors, and drags something around. It last for about 10 minutes. It used to be about 25 minutes long but everytime we listen to it, it gets shorter and shorter. A religious person that has listened to it called it a "devil dog" after my boyfriend and his sister. After it done it cuts right back into them playing. What do u think this is? And does anyone know of anyone in Minnesota who is quallified to listen to it, so i can know what it is?