Mowu

Mowu is a legendary dog from the Plane of Mountains and Seas, and the travel companion of the planeswalker Jiang Yanggu.

Description
The sapient Mowu has two possible appearances: a small Blue Heeler like form, and a larger form with three tails. Mowu can follow Jiang during a planeswalk. This is an exception to the common rule that planeswalkers can't bring living organic matter with them. Jiang Yanggu's only explanation for this is that Mowu is a magic dog, while Mu Yanling has suggested that Mowu is actually made of rock. A change into stone was also witnessed by Araithia Shokta, when Yanggu and Mowu planeswalked away from Ravnica.

War of the Spark
Yanggu and Mowu were lured to Ravnica by the Interplanar Beacon, and immediately became trapped there due to Bolas's use of the Immortal Sun. They actively participated in the fight against the Dreadhorde.