What instrument did John Bonham use instead of his kick drum for the main riff of "Kashmir"?

A timpani (orchestral kettle drum). Bonham used it instead of his kick drum for the main riff, playing it with mallets. That deep, rolling, almost ominous sound is all timpani — not a standard drum kit. He tuned it low and used it like a kick drum on steroids, and the sustain and pitch bend are what make “Kashmir” hit different.