Sounds like it might have something to do with messages that perhaps are not getting through to you in the normal way; partly because of the mailbox bit not being a proper mailbox but a trash can. Also, I don't know if you are aware of this, but people used to use animal entrails to read messages about the future - if you didn't know that it could mean something else entirely. Are you particularly fond of that type of dog? Dogs are associated with loyalty and life-long friendship, so it could be that you are unconsciously examining a friendship or friendships, looking deep inside them. Sounds kind of yucky as an image, seeing the guts of dogs, but the subconscious doesn't always see things the same way - it's more likely that it's trying to get you to look inside whatever it is that these dogs (or any dogs) mean to you, which for most people is a loyal companion.