THE CHEE
The Chee are androids and very advanced when it comes to technology. There true shape is a metal wolf standing on two legs but the Chee project holograms of anything around themselves to hide them. They are very strong but programmed against non violence. They Pemalites (the Chee creators) where capable of faster-than-light space travel were the Andalites were still learning to make fire and when they first visited Earth, humans were only hairy apes.
Their creators, the Pemalites, were creatures of peace as any evil in them had gone as they had been in existence for so long that it had disappeared from them. They built the Chee originally as toys but eventually they become the Pemalites companions. The Pemalites programmed them to be non violent. Chee was a word that meant friend.
The Pemalites loved to play games, they loved jokes and loved laughter. everyone was friends and there was absolutely no evil.
Then one day Howlers (creatures of Crayak) came to their planet and killed almost all of the Pemalites as they were defenseless because they had forgotten how to fight. The Chee escaped with a few hundred Pemalites but a killing germ had been released and when they reached Earth (they had chosen to go their as they thought it would be safe) their was only 6 Pemalites alive.
By now humans were building cities and factories and after they died the Chee took the Pemalites essence and put it into wolves and dogs as that was what the Pemalites vaguely represented.
From then on the Chee would pose as a family of humans and slowly let their hologram die before coming back again as another human. They also had an underground park like place were they looked after dogs, as they loved dogs because of the Pemalites. They also had a Pemalite ship that was hidden under the sea where no one but a Chee can reach it.
The Chee worked against the Yeerks, but they did it their own way by gathering information and passing it onto the Animorphs hoping that they'll be able to use it against the Yeerks.
For more about the Chee read #10 The Android or #27 The Exposed.