I found out how to get my custom entity to replace the vanilla EntityFox. This should fix most of the errors people were getting.
I also rewrote the entire plugin. It saves some data with SQLite and some parts of it are cleaned up, optimized, or certain things moved in an appropriate class.
Updated for 1.15.1 (see "1.14 TRANS.xlsx" for translated methods from 1.14 to 1.15)
Added naming (after taming) supporting color codes
Changed formatting of fox names. ("(name) (tamer)") - can be disabled in config.
Showing of who tamed can be disabled in config.
Improved usability of /spawntamablefox + added alias of /tamablefox
Configurable option for tamed foxes to attack chickens and rabbits.
Customisable plugin prefix