Bettas are the (in)famous Siamese Fighting Fish. They will attack other bettas and fight them to the death. They have also, from time to time, mistaken other fish with lots of finnage for their own kind. So, fancytail guppies, fancy goldfish, Lyretails mollies etc. can be subject to this aggression. I would advise you not to add a betta to a setup with guppeis already in it.