You've trained Sea Serpents, so that's probably trivial for you, but how would you set up an auto-butcher device? I can see a 5Z-level pit with a butcher's shop at the bottom, but how do you knock wild animals into it? Cage traps, pit's-edge animal stockpiles, and pitting? You speak of automation, so I'm imagining something with no dwarven intervention...
Catch wild animals in a cage trap. Get them into a confined area by any of a number of methods - build their cage next to a shallow breeding pit and designate the animal for pitting, or build their cage in the confined area then open it with a lever.
For a simple animal breeding system, all you need to do is leave male and female wild animals in a sealed room for a while. Wait till there are more of them. Open a door from their confined room to the outside that leads through a corridor with enough cage traps to catch them all. Place a male and some females back in the room and kill the rest.
If you want to get ambitious, it's possible to use an elaborate system of pressure plates, floor hatches, and water logic gates to do this automatically. Build a machine that passes the wild animal herd through a counter, diverts a fixed number of adults back to a breeding chamber, and then sends the rest to a retracting bridge over a deep pit. I had a machine that did exactly this in my last 40d fortress. If wild grazers don't need to eat in the current version, I might recreate the design in my next .19 fortress.