Answer:
There are several different recipes for Haggis, the basic ingredient is minced Sheep's pluck, which is the heart, liver and lungs of the sheep; mixed with oatmeal, suet, and diced onion. Traditionally this was stuffed into a sheep's stomach and cooked; modern haggis is usually made in a casing instead. The amount of ingredients varies considerably depending on the recipe you use.
There is even a vegetarian version of haggis