When finishing the Paradise Lost mission in the Grimvale Quest, you are rewarded a toy ball from NPC Niral.
Once you get the item, simply go in the south part of Oskayaat, where you have a big tower with a sphere on the top, then step on a cracked tile (here) to go to a lower floor. There you'll find a foxmouse.
Here you just use the toy ball on it and you'll tame it!
Note that you need to be free from the were sickness to be able to tame it, so in case you use the toy ball and it doesn't work, you'll have to use purple nightshade blossoms to get rid of the curse first.