This hen is a broody girl — meaning she has a strong instinct to incubate eggs, even if they are larger than herself. The Twins painted this hen in rainbow colors for Thawfest and for the Norse god, Loki.
The Thawfest Hen appears to be a typical Berk female chicken of unknown original color. Currently she is rainbow-striped after being painted by Ruffnut and Tuffnut.
Thawfest Hen may be a reference to the episode, "Fright of Passage" when Tuffnut wanted a rainbow-painted chicken from Snotlout.