The protagonist's first look at an alien is very important for setting up first impressions. It is here that we learn a key physical feature about them that makes them different, how they behave, and how the character feels about them.
He edged a little closer to see what they were attacking.
Then it roared and reared up on its hind legs, scattering the men –
– and Lucky saw horns, great curving horns, pointing up at the sky –
– and his stomach turned to liquid. His blood ran cold.
Because it was an Alien. The first he’d ever seen in real life.
From this, we know the Aliens in Phoenix have great curving horns, roar animalistically, and invoke fear in humans.
What kinds of aliens appear in the examples below?
He adjusted his position to see a little better, all concern about being caught as a stowaway forgotten.
The creature roared, unfurled its tentacles and tried to swat the shooting soldiers –
– and Biff saw teeth, rows and rows of teeth, going down its throat as far as he could see –
– and his throat went dry, his eyes nearly popped out of his head.
The creature was the most horrifying thing he’d ever seen.
She hit the brakes and looked through a scope.
It rose from the desert floor, sand streaming off its back —
— Vijay saw black legs, shining black pincers, glistening in the sun —
— her heart beat faster. Her skin prickled.
She had woken a saffak. A desert hunter.
She peeped over a row of coral to catch a glimpse of it.
Was it one human? Ten? Attached to a machine that propelled it (them?) as fast as any fish –
– and Jola saw spears, long metallic spears, aimed at a school of parrotfish just going about their lives –
– and her heart began to pound. Her body stiffened in fear.
This was the Enemy. Far more terrifying than the briefings and training exercises implied.