This is true. Xerneas has two forms, however, they both look almost the same. Xerneas has a Neutral and Active form. The active form is the one you see in battle, and the neutral form is what Xerneas looks like outside of battle. In its neutral form, it looks almost the same as the active one, except the yellow parts of its body are a light blue colour, and it doesn't have any bright colours on its antlers.
The neutral form actually isn't very special or cool, since you can't use it in battle and it doesn't have any stat changes or important things like that.