Some of the stations, possibly the Orchid possess many other white bunnies, as implied by the fact that the number 8 is painted on the rabbit's back.
The animals are related to the act of trying to change the Valenzetti Equation. A theory has developed that numerous white rabbits on the Island were labeled with the numbers 4, 8, 15, 16, 23 and 42. Several rabbits could have the same number, but all the rabbits seen so far have the numbers 8 and 15.
The bunnies are all left-overs from the zoological DHARMA experiments.
The experiments were based on life extension, much like Joop, which explains why there are still numbered rabbits on the Hydra Island.
Rabbits are often used in laboratory experiments as they have high reproductive rates (4-10 litter size) and a short gestation period (31-32 days). This would make them particularly useful in Eugenics studies. Their advantages as laboratory animals include good access to blood vessels, size, reproductive rate, and suitability for cage housing. The #8 painted on it's back is further indication that it was an experiment's subject.
The bunnies are a food source.
The bunnies are pets.
The bunny number 8 was Ben's pet, and that's why he seems so affectionate towards it in the end of Every Man for Himself.
The rabbits are albinos, and albinism is a recessive trait. The genetics of the situation, coupled with a numerical designation, may point towards cloning - either by the Initiative as part of their life-extension program, or by the Others.
Much like Daniel's rat, Eloise, the bunnies were used to test being "unstuck" in time.
Ben asks if the bunny they ate had a number on it suggests that eating one of the possible labratory bunnies is harmfull or will have some side effects, and that ben is aware of it.
The rabbits, like Desmond are subjects of Time Travel experiments which would explain why in the Orchid orientation video a second rabbit number 15 appeared and why Marvin Candle was so adamant about keeping the two of them apart.
The rabbits are used for time-travel experiments as seen on the "Orchid Orientation Film". We know the Orchid is for time-travelling experiments and that no non-organic matter should be put in the machine. Also when Benjamin Linus brings rabbit #8 to convince Sawyer to obey him, he brings it back implying that he never killed it. In fact, he just used the time-travel machine to have what seemed a copy.
Connections to the March Hare from Lewis Carrol's "Alice's Adventures in Wonderland" (i.e. "Follow the white rabbit")
Rabbits were killed in an early and well-known pregnancy test. The urine of pregnant women contains human chorionic gonadotropin, which causes changes to a female rabbit's ovaries when it is injected. Early pregnancy tests involved injecting the urine and then killing the rabbit and inspecting its ovaries.
Alex was dissecting a rabbit in "Greatest Hits" to see if someone, possibly Kate, was pregnant.
Karl asks if Alex brought rabbit for him for food.
Because the Others/Dharma with their vast resources can't get newer means of pregnancy testing?