In hindsight, I've come to realize that Cow and Chicken's parents being only legs was actually the worst-kept secret of the entire series, because of just that: it really wasn't a secret.
There were hints throughout the series that they were just legs, ranging from the fact that their shadows were always cut off at the waist, and when Cow tried masquerade as both Mom and Dad, she only wore Mom's dress and Dad's pants. The series finale even revealed that they were only pairs of legs, prompting Chicken to remark to Cow, "Look on the bright side, at least we're normal."
But again, the reveal was so anti-climatic because we already knew they were only legs for two reasons: 1. The "No Smoking" pilot did the exact same reveal in the end, and 2. Actual promotional material showed they were only legs. So really, all of that hinting throughout the series and the reveal in the series finale built up to absolutely nothing that we didn't already know . . . it'd be like no mystery to what's under Double D's hat because it was already revealed prior to the series being released anyway.