I mean they’re only weird because we’re not used to them, giraffes, narwals and sea-urchins are pretty weird as well.
Not to mention something like the platypus
Even snakes are bizarre. We have a creature with no limbs, just a very dangerous head and a potentially very dangerous body, and it uses its skin to move. And they can eat things whole which are several times the size of their head. Seriously, wtf.
Oh and even better, they range in size from adorable little worms to big enough to eat a human whole. And what kind of exercise do constrictors even do to get strong enough to suffocate something that outweighs it??
Even weirder is that snakes evolved from critters with legs, and multiple times independently. They were like “legs? We don’t need no stinking legs.”
Wiwaxia, Hallucigenia, Tully Monster and Anomalocaris if anyone’s wondering
Anomalocaris means “weird shrimp” so I get the feeling the Cambrian freaks out the nerds sometimes, too.
My favorite ancient critter (actually pre-Cambrian) is Omnidont which means “all teeth”.
Why did swords sticking out of brain need to evolve?
Great question! Do some science.
I’ll give you a starting place. Consider their mobility, camoflauge, protection, abilities of their predators, habits of their prey, and mating strategies.