first of all 2000 gems for ONE skin is just ridiculous. and on top of that 30 rng skins for 120€ just WOW. how low can arenanet get with this game. barely any content but gem store shit everywhere.
this happens mostly when you just waypointed . it hasn't loaded the mount 'files' yet . this gets worse the slower PC you have. solution? wait a bit before mounting after waypointing or just mount and wait till your mounts shows up