This pet/toy would be about the size of a softball, and be covered in fur the color of your choice. It would be placed on a desk and just sit there, waiting...
Of course it probably would have other features, such as a voice activated alarm clock (it would only be an orb, no eyes, only an orb
of fur) or radio, but this is not its main purpose.
The "Predator Tribble" would sit there, waiting for its oportunity, when it sences the moment is right, the time when you least expect it, (determined by a small clock and a random sequence generator) it attacks towards the nearest sound! Nothing serious, it just hops towards you and possibly bumps you, but it is soft, just enough to stun you and make you think "What the?! Oh, heh, almost forgot about this."
You reset the simple spring mechanism, place it back on your deask, and eventually forget about it, and when that day comes... THUMP! when you least expect it.