Archive diabdat.mpq which is located on the CD with Diablo 1 contains many unused dialogues. After analyzing these sound files, it was possible to recreate some of the quests that were not implemented in the game. This is an evidence that originally the game was meant to be much bigger than the final version of Diablo, however, due to some reasons these quests have never made it into the game. Perhaps the developers ran out of time or the quests were difficult to implement so the decision was to remove them.
In addition to quests, diabdat.mpq also includes gossips about priest Tremain, characters speeches and dialogues by NPCs. There could also be more animals in Tristram than cows, as the archive also includes sounds like duck quacks, pig oinks, donkey brays, dog barks, chicken clucks, and sheep bleats :).