Snakes are for his devil/sin part, and of course Lucifer (They're white to make a reference to the white suit Lucifer!Sam was wearing). The dog head is

The dog placed at the top of his head embody the way he's thinking and the snakes are at the bottom of the composition to represent his dark side.





Thanks a lot buddy! And it's a pleasure for me to explain it:Sam :for loyalty, strentgh, wildness and protection instinct.Dean :The billy goat is to make a reference to his demon part (spoil season 9). Red eye emphasize this demon part. The tiger embody his true nature : hunting, strenght, protection and solitude.The billy goat is placed on the top because his mind is completly infested by his demon part, so it's the way he's thinking for the moment. The Tiger represent what he really is inside his heart.Castiel:The eagle is a reference to his wings. But the choice of the race is not completly innocent... It's an Aquila AudaxTalons represent the strenght of Castiel, his warrior part. Talons on the left are placed like an infinite symbol to represent the life expectancy of an angel. The deer represent his combativeness part to defend his friends and his family. Like Castiel loses his powers, a deer loses his antlers.Here is it! I'm really really glad you asked me for this!