A hockey puck, because of its shape and size.
Joanne got clocked by a horse pill at the Pirates' game last night.
Classically, refers to size; an unusually large pill.
Realistically, should refer to Premarin, the birth control that consists of "conjugated estrogens" extracted from horse urine!
1. How on earth can you swallow that multivitamin; it's a horse pill!
2. She's on horse pills