Introduced during the Masked Memories event.
Obtained by giving Tsua Kagemata a Cow Head which is dropped by a Cow, an easy place to find cows is the cattle farm near Lytelthorpe at 0.1N, 52.6E.
You give Tsua Kagemata Cow Head.
Tsua Kagemata tells you, "You want me to make a mask from this?"
Tsua Kagemata tells you, "Very well... Here you go, mighty hunter."
Tsua Kagemata gives you Cow Mask.