Guruko

is a ninken, and is a personal summon of Kakashi Hatake. He has whisker marks similar to Naruto Uzumaki.

Land of Waves arc
Kakashi called on Guruko and the nin-dog pack to track and capture the missing-nin Zabuza Momochi. They managed to bite onto the missing-nin and pin him in place. This allowed Kakashi to use Lightning Blade without fear of a counter attack.

Sasuke Retrieval arc
Later, Guruko and his fellow nin-dogs were summoned by Kakashi to track down Naruto Uzumaki and his team who had gone off in pursuit of Sasuke Uchiha.

Rescue Gaara arc
At the start of Part II, Guruko and the nin-dog pack were called upon to track down the Akatsuki member Sasori using a small piece of his clothing.

Hunt for Uchiha arc
Some weeks later, Kakashi summoned the pack to help track down Itachi Uchiha. He was partnered with Shino.