פירוש על קידושין 139:6

The rabbis offer creative interpretations of who these people were. The first category were people whose deeds were like those of Sodom who acted licentiously. These people are assumed to be mamzerim. The other categories are people who do not know their lineages. These two are listed in the mishnah about who came up with Ezra.

This midrash plays on the word “Immer” which reminds the rabbis of the leopard. The leopard was not viewed favorably by the rabbis because they thought the leopardess would lie with other leopards she was not supposed to be mating with. I’m not sure why the rabbis expected monogamy from animals, but they seem to have pointed the figure at the leopard in particular.