In Metroid lore, the Chozo were an ancient and proud race of bird-people, who lived on planet Zebes. They considered themselves to be the "protectors of the galaxy", and I believe they were the power behind the formation of the Galactic Republic. When Samus' home on Star Colony K-2L was destroyed by the Space Pirates and her parents killed, the Chozo took her in and raised her. (They also infused her with Chozo DNA which gave her super-human strength and resilience.)
By the time the events of Metroid take place (particularly the Metroid manga, which is at the beginning of the timeline), the Chozo are a dying race; they were able to lengthen their lifespan past their fertility period, so their numbers slowly dwindled, as none of them could reproduce. All their infrastructure and other needs are maintained and governed by this sapient super-computer that they created, known as Mother Brain.
The Chozo were monitoring the planet SR388 to examine a creature that caught their intrigue, known as the horntoad. On that planet they discovered a sort of "slime" that mimics the attributes of organisms it encounters and carnivorously eats everything and everyone. They called this the "X" virus. They knew that the Galactic Republic could not know about this, lest it become a weapon. To destroy this X virus, they genetically engineered a creature known as the "Metroid", which slurps up the life essence of living things.
Mother Brain, the computer that they built to maintain their planet, betrayed the Chozo by allowing the Space Pirates to pass through the planetary defense barrier. She believed that she was the singular "perfect" organism, and that the Chozo were "obsolete". She wanted to use the space pirates as minions so that she could conquer the Galactic Republic and bring order to the galaxy, because only the perfect organism should rule the galaxy.
The Chozo had a psychological barrier that prevented them from harming any form of life; it caused them serious mental harm to do deliberate damage to another life form. When Gray Bird attempted to destroy Mother Brain, he almost died doing it solely from the psychological stress of using a weapon against her. It's been a hot minute since I've consumed this piece of lore, but I seem to remember that he died in a cave-in and didn't get to finish destroying her.
All of the Chozo died while evacuating the planet.
You know those weird statues in the Metroid games? Apparently those are Chozo statues. I always thought the beak was big tusks, and they were alien walrus-esque creatures. You learn something new every day, I guess.