My one guppy was like that, he'd chase anything that moved really if he thought he'd get some nookie and he'd switch from chasing my females to the other male, then the other male would turn and look at him like 'what the hell do you think you are doing?' and then he'd stop, turn around, and go after a female again. He was the horniest guppy out of the whole bunch, I even caught him chasing the pictus once.