The viceroy is a species of butterfly that protects itself from predators by looking like the poisonous and foul tasting monarch butterfly.
This type of imitation of one species by another is called mimicry. It benefits the viceroy greatly to look like the monarch butterfly because predators usually avoid monarch butterflies if they had any previous encounter with them.
Viceroy caterpillars mostly eat the leaves of willow trees thus they tend to accumulate salicylic acid in their bodies which gives them an unpleasant taste and acts as an irritant. Which is nothing nearly as serious as the poisons that the monarch butterflies accumulate from their host plant the milkweed.
Some researches suggest that because both the monarch and the viceroy butterflies have foul taste they both benefit from looking like the other and the mimicry is mutual.
Viceroy butterfly – Limenitis archippus (Cramer).
Limenitis archippus on Butterflies and Moths of North America.
A theory about a more complex mimicry relationship between the viceroy and monarch butterflies: Mutual Mimicry: Viceroy and Monarch on the Britannica Blog.