Agrajag is a creature that gets repeatedly reincarnated only to be killed in various ways, usually by accident, by Arthur Dent. We meet Agrajag several times before we learn who he is.
Wikipedia's got a nice little overview of Agrajag's storyline:
Agrajag is first identified in the novel Life, the Universe and Everything, but it is revealed that several of Arthur's encounters in the first and second novels (and in previous chapters of the third) were with previous incarnations of Agrajag. The first occurs in the novel The Hitchhiker's Guide to the Galaxy, when a bowl of petunias is suddenly yanked into existence miles above the planet Magrathea, and begins falling, having only time to think, "Oh no, not again," before crashing to the ground. The reason behind the bowl's lament is revealed in the novel Life, the Universe and Everything, when Agrajag identifies the bowl of petunias as one of his prior incarnations, and tells Arthur that he had seen his face in a spaceship window as he fell to his doom. In another incarnation, Agrajag was a rabbit on prehistoric Earth (during the time period recounted in the novel The Restaurant at the End of the Universe) who was killed by Arthur for breakfast and whose skin was fashioned into a pouch, which is then used to swat a fly who also happened to be Agrajag. In yet another, near the beginning of the novel Life, the Universe and Everything, Agrajag is an old man who dies of a heart attack after seeing Arthur and Ford materialise, seated on a Chesterfield sofa, in the midst of a match at Lord's Cricket Ground.
This list does not claim to be exhaustive, and I have a hunch that there are more Agrajag sightings around the HHGG universe - even if they are not in the books. Can anyone find any others?