In describing his pet’s lifestyle, the speaker accidentally said that his dog was healthy as a clam. This is a nice mashup of “healthy as a horse” (very healthy) and “happy as a clam” (very happy). The words healthy and happy are similar in sound and shape, contributing to the mental mix up. Happy clams seem to be on people’s malaphor minds, as a prior one posted illustrates – “happy as a clam in clover”. https://malaphors.com/2014/08/15/im-as-happy-as-a-clam-in-clover/. A big thanks to Johann Chancey for saying this one and sending it in!