Answer: Animals were drawn on walls of tombs in the belief that they would come to life my magic in the next world and supply the deceased with the riches and comforts of life that he enjoyed when he was alive. To the Egyptians death simply transformed them into another level of existence where, depending who they were in life, would continue to be supplied with the food, drinks, cattle, workers, etc which they had in their former existence.

I found one example of a cow with her tongue hanging out. This was because she was giving birth in the fields where she was being worked by the field workers and the overseer. This is in the 6th Dynasty tomb of Ti at Saqqara (pronounced Sakkara) - a beautiful tomb to see if you are willing to trudge a couple of km over scalding hot sand in the Egyptian noon day sun - which is about the time you get there after seeing the Step Pyramid etc.

Another example I found was in the weighing of the heart scene at the judgement before Osiris. The deceased is lead before Osiris for judgement. His heart is weighed against the feather of Maat (the goddess of truth who is often represented by this feather - the hieroglyph for her name is as ostrich plume). The heart was thought to be the centre of thought, memory and personality. So to judge whether the deceased had been a good, truthful person, the deceased's heart was weighed against the feather of Maat. If it was as light as the feather the deceased was judged to be eligible to go to the next world and was assigned a portion of the domain of Osiris. If the heart was heavy with sin it would be heavier than a feather and the deceased would be fed to Ammit - the devourer. This is the animal shown in some of the weighing of the heart murals as sitting or crouching beside the scales with his tongue hanging out awaiting in anticipation for the chance to devour the miscreant deceased. Amit had the head of a crocodile, the front of a lion and the hind quarters of a hippopotamus - all killers.

Animals whom the Egyptians deemed to be sacred and the animals forms of gods and goddesses do not seem to be depicted with their tongues hanging out. Nor do animals working normally in the fields. The pictures of the horses in the Ramesses II chariot scenes do have their mouths open as the bit is held taut by the reigns tied around his waist. I haven't seen every tomb mural depicting animals in various actions, so I can't say these are the only instances of animals with their tongues hanging out. So if you care to email me a specific image which does not fit the answers I have given you, I will be glad to check it out for you. For the present I would say "look at the whole picture - is there action where the tongue would normally be hanging out in everyday life?" These tomb murals were meant to come to life for the use or benefit of the deceased, so they would normally be everyday actions. The scenes from the Book of the Dead are different. They are the series of tests the deceased has to contend with to get to what we would call heaven. So they can be filled with danger like Ammit - The Devourer salivating at the thought of a tasty bad guy who couldn't be justified as a truthful, fair good person.