The Al-mi'raj is a fantasy monster and a prime example of why TSR's Fiend Folio tome wasn't very useful for most AD&D players. This monster seems to have been strangely named after one of the Muslim words for heaven. Now, that could have been something interesting, although I prefer to keep anything related to living religions out of roleplaying due to common sense. But instead of an interesting beast, we get a slightly oversized bunny with a unicorn's horn.
The authors could have made up for the poor concept with a great description, or some other play hook. But they chose not to. Instead we get a few almost useless statistics, and a few lines about these beasties roaming pastures and woodlands. With a minor note about them being unpredictable. Apparently they can be captured while young and can be trained to become companions. The companion angle might possibly be the source of the monster. Perhaps some minor player in some campaign run by a TSR staff member had a trained Unicorn Bunny, and they subjected the rest of us to it as well.
But once you start thinking about it, it seems to me that the producers of the book had a picture of a "Unicorn Bunny" laying around, so they wrote a few lines to go with it, then called the results a "high quality presentation". This is a problem with the Dungeons and Dragons monster ranks in general, but this version in particular. There is simply way too much overlap amongst types of monsters. The Monstrous Manual alone contains something along the line of 20 or 30 different humanoid races of the orc and goblin variety. If you add the other monster books, and then count all the ones added in other TSR publications the number shoots up to being hundreds of them. Equally there are hundreds of varieties of Dragons, oozes, undead, and generic bear sized monsters. It really just gets crazy. But at least you might have a reason to want statistic for 197 different types of zombies. I cannot however, understand why any dungeon master would feel the need to add unicorn bunnies to their game.
Of course I should provide some kind of description, just in case you are aching to use this bunny in your fantasy world. The Al-mi'raj is a semi-mythical subspecies of rabbit that has developed a single large horn on their head. These large rabbits average about three feet long, plus an additional 1-2 feet for the horn. Like any rabbit, the Al-mi'raj can be unpredictable, so beware. These monsters do not carry treasure, so they are best left unmolested.