I think puppy hybrid dennis is a serial tail chaser- specifically in the sense that he chases his own tail every time he's caught off guard by it.
if he's laying in his bed snoozing and suddenly his tail twitches next to him? hes leaping up like hes been stung to try and catch the mysterious moving object, snapping his jaws and circling over and over to try and catch up with it.
if he sees it wag in his peripheral vision as he's playing? he's abandoning his toys to leap and bend himself backwards, growling and barking in pursuit of the elusive golden hair. he never quite manages to clamp his sharp canines around it, and fails at his hunt every time.
when robby watches his pup snarl and snap and spin over and over again, dennis convinced he's going to end victorious and with the strange creature behind him subdued, he wonders how his puppy can be so smart and so stupid all at once.
he usually has to grab dennis to stop him, physically grasping his tail to show him that it's one of his own appendages ("look, denny! puppy, look, it's attached to you. it's your tail, baby. you can't hunt it."), and then watches as dennis slowly understands, letting out huffy little puppy grumbles before he curls up again for another nap. he'd be back "hunting" again soon, of course.