Purim Costumes Hinted To in the Gemara

Halachicly Speaking (h/t Avodah):

The custom to get dressed on Purim is not mentioned in the Gemorah, medrash, or the Geonim.

Nevertheless, there is a nice hint to it from Shabbat 66b where Rashi explains the word פרמי (a word resembling "Purim") as טלמוסא הנקשרת על הפרצוף להבעית הבנים קטנים (masks tied onto one's face to scare small children). 

ברוך שכוונתי as I see that this was mentioned in the link (footnote א - reason 8) that I mentioned previously.