When discussing anatomy of any animal, it's important to first gain a point of reference and establish terminology for description. It's fine to say that the trachea is in front of the esophagus when we are looking at a penguin from the front, but the esophagus is in front of the trachea if we look at the penguin from behind.
Vessels can run under a tissue from one point of view or over a tissue from another point of view.
Therefore, to be consistent, we establish terms that apply to location from any point of view. The Adelie penguin on the left of the screen is bisected by three planes. You can see a plane running parallel to the ground, running right through the abdomen of the penguin. This is called the horizontal or transverse plane. However, all horizontal planes are parallel to the ground.