A rabbit warren is an artificial enclosure for raising rabbits.
The rabbit was also introduced into England, probably from Iberia, by the Normans for their fur and meat. Originally rather delicate creatures in order to thrive they had to be sheltered from the elements, protected from predators and fed regularly. To facilitate this artificial warrens were built consisting of low flat topped mounds, 10 to 20 metres long, 5 to 10 metres wide and up to a metre high sometimes surrounded by a shallow ditch. Such mounds are often called pillow mounds [...] Warrens sometimes of considerable size existed on Dartmoor and around Thetford into the early twentieth century. Such large establishments boasted extensive banks to enclose the warrens and other features including vermin traps and pitfalls for catching the rabbits.
Source: The Archaeology Of Hunting.
The word warren also refers to a colony of wild rabbits or other rodents with many holes, often interconnected.
Thus a rabbit warren can be a system of interconnected rabbit holes and tunnels, or a figurative term for something complicated.
Possibly the "rabbit hole" in:
You can't divide them, but that's a rabbit hole of its own.
means a rabbit warren, used figuratively to mean something complicated.
Thus it is possible that in this case "rabbit hole" = "rabbit warren" = something complicated.