Old Fourlegs
/ˌəʊl(d) ˈfɔːlɛɡz/informal
noun
(A nickname for) the coelacanth, Latimeria chalumnae.
Origin
1950s. From old + four + the plural of leg, with allusion to the former belief, later shown to be unfounded, that the fish walked on its four leg-like fins.