humans
In one of the books in The New Prophecy some of the cats get taken from twolegs
Yes, Twolegs occasionally come into Thunderclan's territory in both the original and the lake territories.
Yes, Cloudpaw returns from the Twolegs in the "Warriors" series by Erin Hunter. After being taken by Twolegs, he eventually escapes and reunites with his clan, ThunderClan. His experiences shape his character and contribute to his development as a warrior.
He was helping Squirrelflight, and some other cats of ThunderClan rescue the cats that the Twolegs had trapped, like Mistyfoot, Leafpool, and Gorsetail. He stayed behind to help open Brightheart's cage, and the Twolegs grabbed him.
Humans (Twolegs)
He died of poisoning sickness that was caused by the twolegs.
Cloudpaw, after becoming Fireheart's apprentice, proves to be extremely stubborn and selfish, and later betrays the warrior code by allowing Twolegs to feed him. Later, he learns his lesson when he is captured by the Twolegs and stolen from the forest. Fireheart, Sandstorm, and Ravenpaw rescue him, and he proves to be a diligent, hardworking apprentice from then on. Bluestar makes him a warrior, Cloudtail. Later on, he becomes the mate of Brightheart and has one kit with her, Whitewing.
In this book, Graystripe is still prisoner with the Twolegs.
She died of poisoning sickness caused by the twolegs.
In the Warriors series, humans are referred to as "Twolegs" by the cats. They are seen as large, noisy creatures who sometimes pose a threat to the cats' way of life by destroying the natural environment or encroaching on their territory. Twolegs are generally viewed with distrust and are to be avoided by the clans whenever possible.
No claws are humans. Like in warriors they are twolegs.
Graystripe met Millie at a Twoleg's place when he was captured by Twolegs.