Fieldbreakers are large, rotund Rabbid enemies covered in Darkmess that appear in Mario + Rabbids Sparks of Hope, specifically in the The Last Spark Hunter DLC and the Rayman in the Phantom Show DLC. In the former, they first appear in the Missing Musicians battle on the Melodic Gardens, and in the latter, they appear in the All Hands Ahoy! and Burnin' Rock battles. Their appearance is similar to that of a Buckler from Mario + Rabbids Kingdom Battle. They have a cannon on their right arm.

Source: https://www.mariowiki.com/Fieldbreaker