From Muppet Wiki

The Humongous Chicken is a giant, slow-witted chicken (referred to by Huxley as "Tiny") who pesters Elmo as he tries to get to Huxley's castle to retrieve his blanket in The Adventures of Elmo in Grouchland.

The chicken is a normal-sized hand puppet (which has also been used on Sesame Street with feminine eyelids), but is superimposed over the landscape to appear bigger. He is one of Dave Goelz's few Sesame characters.