Mobility would be simple, you basically just make the costume be like a ballroom gown in that it covers all the way down to your feet but it has a hole where you walk/you're feet are and thats the point where the costume continues backwards if you want it to trail behind you.
How big you made the hole would be your own choice; as for materials I would just lookup some basic mascot costume tutorials since they would have the best suggestions since he looks somewhat like cloth/mascot like.
Cosplays in progress
Phayne/Henger (Monster Ranher 4) - Completed (Katscon 2014)
Blastoise (Full Scale) - Construction phase
Poe (Legend of Zelda) - Completed (AnimeNext 2013)