The marking on Fenn Rau's helmet are early Joe Johnston designs for Boba Fett's Helmet created for The Empire Stikes Back.
Mandalorian Protector Fenn Rau was named after episodic director Brad Rau by Dave Filoni (source: Starwars.com)
The Battle of Mygeeto, in which both Kanan and Fenn Rau participated, is depicted in Marvel Comic's Kanan issue 10, published January 2016.
Rex mentions that the Mandalorian Protectors were recruited to train Clone Troopers during the Clone Wars. This is a reference to the Mandalorian Cuy'val Dar ("those who no longer exist") who trained Clones in the Republic Commando novels by Karen Traviss.
Despite being the homeworld of famed bounty hunter and clone army progenitor Jango Fett, this is currently the first and only canon appearance of Concord Dawn in Star Wars film. As such, it is unknown what Jango Fett's affiliations were with the Protectors.