I heard the following from Chaim Channan who heard that the Kotzger Rebbi said this (paraphrased by me):
What is the Animal Soul? An example could be noted when a horse goes to drink some water. When the horse gets to the water it stomps it’s feet. Why does it stomp it’s feet? Because it sees it own reflection and thinks that there is another horse there trying to get the very same water that it is trying to get. Once the horse starts drinking it stops stomping it’s feet because it realizes that there is enough water to “share” with it’s reflection.
When a person acts in such a way – to feel frustrated at the prospect of having to share something that may seem limited, even if once they start sharing they realize there is enough and stop being frustrated – it is apparently the Animal Soul. Something nice to keep in mind to keep our Animal Soul in check. We have to use the G-dly soul to overpower this inclination and be instantly giving without frustration.