There were a couple of misunderstandings when I was a kid.
When I watched the first LBT film the first couple of times, I thought Littlefoot's mother wasn't his biological mother. I thought the egg rolled too far away from the longneck in the pond and into a different longneck herd that just took him in.
Second, I saw LBT 5 before LBT 2, and again, didn't think Chomper's parents were his biological parents. In fact, I thought it was the plated sharptooth that was related to him. Why? Because Chomper and the plated sharptooth had three fingers, while his parents had two.
Also yeah, I thought Tippy was a girl. And on first glance, Rinkus. Reverse colour coded for our inconvenience.

Pretty sure I thought Littlefoot was a girl at some point, too.