A cat randomly appears and disapears from the Google Search bar. What is this? It looks like the Cheshire cat but I was unable to find anything on the internet about it.

This is a Pixel 2 phone with stock android.
It's a doodle from Google related to Sir John Tenniel's 200th birthday. This is a feature for Google search app widget which you can also disable in Google app settings. You can read about the feature here.