Mini-Luggages adopt a variety of shapes, depending on who or what their name is derived from. Igor's minis, for example, are various doctor's bags, while minis relating to the wizards of Unseen University are usually tobacco pouches. The only hard rule is that they are all containers of one sort or another — and that they all have hundreds of tiny little legs.
Like all minis, mini-Luggages enjoy eating raw eggs, bacon, and fangirls. In particular, mini-Luggages have shown a liking for fangirl souls, but fortunately for the inmates of the Official Fanfiction University of Discworld, they can usually be persuaded to give them back.
The Mini-Luggage Adoption Centre can be found here. It includes a full list of known mini-Luggages, and notes on which have been adopted.