Spoilers it's not a real animal
The Wild Haggis (Haggis scoticus) is a creature native to the Scottish Highlands. It should not be confused with haggis, a traditional Scottish dish made from the innards of sheep (including heart, lungs, and liver). Accept no substitutes for the real thing.
The Wild Haggis's right and left legs are of different lengths (cf. Sidehill gouger), allowing it to run quickly around the steep mountains and hillsides which make up its natural habitat, but only in one direction. Owing to a process of natural selection, there are therefore two varieties of Wild Haggis, one with longer left legs and the other with longer right legs. The former variety can run clockwise around a mountain (as seen from above) while the latter can run anticlockwise. The two varieties coexist peacefully but are unable to interbreed in the wild because, in order for the male of one variety to mate with a female of the other, he must turn to face in the same direction as his intended mate, causing him to lose his balance before he can mount her. As a result, differences in leg length between the Haggis populations have become further accentuated with the passage of time.
How to hunt Wild Haggis
First, be sure to identify whether the creature you are stalking is a clockwise or anti-clockwise variety. Dig a large hole in which to hide and then, as it approaches, leap up and yell, "BOOOO!!" in its face as loudly as your lungs permit (it will be noted that facing in the wrong direction at this point would be rather ineffectual, hence the importance of pre-determining the rotational preference of the intended prey). Correct application will cause the animal to take fright, recoil and turn to attempt to run in the opposite direction, the consequences of which will be immediately apparent. While it is still stunned from rolling downhill, simply pick it up (assuming that you have managed to keep pace with its rate of descent and reach it before it has recovered), whack it over the head with your shillelagh and pop it into your haversack.
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Wild Haggis totally exists. There is no way it could be false. Do not question the Haggis.