User blog comment:B214/Volume 22 spoilers/@comment-73.46.6.122-20160906202532/@comment-9595671-20160908223251

I'm not even sure how disguises could work for any supernatural being that's not atleast somewhat humanoid, and Gasper doesn't hide his pointed ears or have had a normal human point them out in present time.

There is Rias' familiar, but I think that ability is anime exclusive. If it's not then she rarely seems to use it, or even use it for anything long-term.