The Colossi are ancient purpose grown monsters of immense size and power.
Colossi are a unique race of monster. Instead of being born, each and every Colossi is individually designed, gestated and then grown. Therefore they all vary heavily in appearance and function. The only common factor is their sheer size. Every Colossi is a towering specimen, standing several miles tall and being able to step over mountains like they were speed-bumps. Probably, every colossi is 6+ kilometers tall.
Their body shapes show some variability with Goliath possessing a relatively lean body against Ymir's significantly stockier frame.
Colossi are an ancient race of created monsters. Their original creator is unknown but they have existed for millions of years.
Every Colossi is built for a purpose so their powers are unique to every individual.