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 Rancher 4) - Construction phase
Blastoise (Full Scale) - Design phase
Poe (Legend of Zelda) - Completed (AnimeNext 2013)