There are some clues

"Grimes shares song about putting her obese cat on a diet"

https://www.nme.com/news/music/grimes-62-1234625

"hi here's grimes booping her cat"

https://www.reddit.com/r/Grimes/comments/9wrppc/hi_heres_grimes_booping_her_cat/

"Grimes Releases Collaboration Song With Majical Cloudz About A Fat Cat –"

https://music.mxdwn.com/2014/07/27/news/grimes-releases-collaboration-song-with-majical-cloudz-about-a-fat-cat/

"Listen to an unreleased song about Grimes’ cat" – Consequence of Sound 

https://consequenceofsound.net/2014/07/listen-to-an-unreleased-song-about-grimes-cat/

Grimes & Devon "Voignamir"

https://youtu.be/YxC3kjPuGZo