It is interesting sometimes how words are paired to describe things. I wouldn’t probably use the word grotesque to describe this letter S drawing but in the world of design that is how it would be described. Christopher Dresser (1834-1904) explains the concept of grotesques and how they were often part of designs. In this case it is a characteristically Celtic design depicting a Letter S bird. It is designed to not appear truly birdlike so as not to make the viewer uncomfortable.
If I didn’t have benefit of Dresser’s description, I admit that I may have missed the pair of feet sticking out to the right and the incorporation of Celtic knots at several of the intersecting lines. This Celtic S image one of several interesting drawings from his 1870 work Principles of Decorative Design.
This image is copyright free and in the public domain anywhere that extends copyrights 70 years after death or at least 120 years after publication when the original illustrator is unknown.