I'm sure that there are planets all over the galaxy where the same or similar creatures evolved and didn't get wiped out but instead evolved into higher animals.
Our lineage was lucky to go on to create humans because all the other ones got wiped out in the Cambrian for some reason.
If those same creatures had survived, they would have evolved into more unique forms of life and we would have called them aliens.
Funny part is, those same creatures I suggested that might exist in alien worlds might one day run into us and look at us like some kind of weird animal that might have evolved out their own planet's Cambrian extinction event.
"They reproduce by doing what? Why? How? Wait almost a whole solar cycle? Give birth? Then they can't take care of themselves? ... How did these beings even survive this long to evolve?"
The biggest fiction (constrained by budget obviously) of shows like Star Trek is that most of the intelligent creatures we might possibly meet will look almost exactly like us. I don't think even the people coming up with Star Wars aliens have the imagination to get it right. They still base it on what we are limited to thinking up as humans and our own likely narrow understanding of what is life and what is intelligence.
The second-biggest fiction is that it would be possible for us to coexist on one planet's surface considering our needs when it came to gravity, atmospheric pressure and basic atmospheric composition would be very unlikely to be the same.
But that would narrow the scope of a lot of sci-fi, so I let it go.
There is a hypothesis that an alien might actually look like a crab. Because life somehow managed to create crab-like creatures from a number of different evolutionary lines. Which shows a certain advantage that seems to be there with this specific form.
Everyone was doing weird shit in peaceful mode, just vibing and experimenting, and then that FUCKER on the bottom right turned on PvP. Being festooned with knives is how they said "I'm tryna get these nutrients, don't @ me."
tbf there are some pretty weird looking creatures in the ocean even now. Like, would the giant deep sea Isopods really look that out of place next to stuff like Anomalocaris? We still have plenty of spiky worm shaped things living on the bottom of the ocean. And for the softer side of animals, would things like siphonophores really look that out of place in a lineup of Cambrian fauna, if placed there and shown to someone who wouldnt have the knowledge to recognize what they actually were?
Cambrian seems like heavens design team didnt have an established meta and were just having fun with it. Once the horse design got approved the engineering team got lazy and used it as the base for everything.
Chordata is the third-largest phylum of the animal kingdom (behind only the protostomal phyla Arthropoda and Mollusca) and is also one of the most ancient taxons.
These are all immensely powerful evolutions. We got the Knifey Tortoiseshell, Cheval De Frisetipede, Sucky Long Mouth Humpback Trilobite and Sabertooth Flatworm. All the greats!
Don't mean to make this all serious but sure you could talk about how god or a designer was just playing around or you could think about how the environment and the selective pressures were different during this period of the planet's history.