Pictograph of a person holding onto a staff, looking to the side with his mouth open, doubtful of which direction to go. The 龰 (foot) component was added later to indicate movement, and the 牛 component was added to hint at the sound.
疑 and 牛 don't sound similar in modern Mandarin due to historical phonetic changes. They were more similar in older Chinese.疑 /*[ŋ](r)ə (< uvular)/ 牛 /*[ŋ]ʷə (< uvular?)/
Due to historical stylistic changes, this component is less similar to 牛 than it was in ancient scripts.