Volt goats respawned after killing all but one, but either there wasn't enough space and two glitched out into the water, or two of them spawned on the water itself. See screenshot.
1. Find existing volt goat herd and kill all goats except one
2. Chase it until it gets stuck trying to run away in a corner of the land
3. Wall it up until it only has a 1x1 space left to itself
4. Leave and wait for it to spawn a full herd
5. Return to observe goats walking on water