When a Xenomorph is made, it comes out of a human body, usually. A face huggers implants an egg in the humans stomach and a baby Xenomorph brakes out. When it's in the body though, it takes some of the humans dna. As though the face hugger was parent number one and the human was parent number two. The Xenomorph takes dna from both.

This explains why they seem so human like. The teeth, five fingers, most of the body. Of course, this means that a Xenomorph would look different if the face hugger implants an egg in a different type of body.

Why the Xenomorph didn't immediately break out of Scar, he had no idea. Hell, he didn't even know it was there. He believed he was quick enough before the face hugger managed to do its thing.

If he had known it was in him, he would've gotten rid of it immediately.

Instead, he got killed by the queen he was fighting. The other yautja, having no idea of the infection, respectfully took his body to rid of it properly instead of leaving it. He was a young warrior who killed a Xenomorph queen, it's the least they could do.

But when on the ship, the baby came out, given it's species, it rapidly grew into a predalien. A mix of a yautja and Xenomorph. Instead of a human and Xenomorph.

And it looked partly like Scar. Given it had Part of Scar's dna. Course it would look so what like him.

This thing was dangerous with both. A yautja and Xenomorph are both dangerous on their own. But them being combined? No one on the ship survived. And with no pilot, the ship came crashing down back to Earth. Letting the predalien free to roam it.