For me, perhaps the most fascinating are the small deer.
Even while I'm striving to keep an eye open for them in the springtime, I usually don't see any until they've doubled or tripled in size from their birth. I would say in humans, it would be equal to discovering them at about five years old.
But even when they're in the pre-teen or teen size, I'm happy to see them. The one following was born in the spring, and I took the picture in September.
Unless they have a strong distinguishing characteristic, it becomes difficult to keep up with them from year to year.
But it's a good guess to assume that the buck you see in the woods in the winter started out as the little guy having fun in the springtime.
Photos from my collection.