Configuration files need to be regenerated.

Usually friendly mobs, such as animals (pig, sheep etc.) have death messages now (for when a player is killed by one). This is intended for mods, custom maps or plugins (such as Revenge by Asynchronous).

Also, some config features have already been added for upcoming 1.12 mobs (Illusioner and Parrot), as well as a config option for showing death messages for named pets (however, this isn't implemented yet, and will be implemented for version v1.12).