1745 Jack n' Tom

It's nice when we use human names for animals that we keep them non-formal. Nicknames. Like the term Billy Goat. Obviously a familiar reference to the male of the species. And only confusing when said goat is owned by a yokel from the hills. Is it then a hillbilly goat or a hillbilly Billy Goat?
There may be other Billy animal references but I'm not sure. You don't hear of a Billy Bear or a Billy Marmot. Some animals don't take to friendly anthropomorphism. It's quite a stretch to say a Billy Lizard. Although I have heard of Ol' Man Crocodile.
Maybe reptiles are better left anonymous.
I've heard of the name Tom used with birds. As in Tom Turkey. It seems to tail off there though, as you don't hear of a Tom Chicken or a Tom Vulture. There is a little known reference to a small bird called a Tomtit. And you may have an encounter with a tom-a-hawk. Tom is not entirely confined to the avian family however. There's also its mammalian familiar, the Tomcat.
Interestingly, you don't hear of a Jack Frog or a Jack Duck. But you do hear of Jack other things. And what an amazing spread. Witness the difference in size between a Jackass and a Jackrabbit. Then there's the Jackdaw and Jacksnipe, birds, and the Jack Mackerel, a fish, and even the Jack Pine, a tree. That Jack guy got really jacked up about naming things. He must have been quite a jackanapes.
Jack, Tom, and Billy, and of course lots of animals are Harry but that doesn't count. But notice, they're all nicknames. A Johnass or a Thomas Turkey would be weird.
And I much prefer "Randy as a Billy Goat" to "Randolph as a William Goat."
America, ya gotta love it.