It's banner #104, item ID 2913. It's listed on the
wiki -- Pre-Hardmode section, second column, 4 rows down. My kill count on my corrupt world currently says only 2. It's possible to obtain the banner, though the worm glitchiness makes it a grind to get. (Side note: This seems to be a left over issue unique to the Devourer. Giant Worms, Diggers, and Wyverns have their respective banners mapped to all corresponding segments.)