The Beeper was one of Man-At-Arms's inventions seen in He-Man and the Masters of the Universe.
When the Royal Zoo was in need of some Wolfbats, Man-At-Arms constructed this beeper to summon them by emiting a high pitched frequency. In fact, the beeper would attract anything that flies, even Stratos of Avion.
Teela was showing her father's gadget to Prince Adam when they heard a loud noise coming from Orko's room. Somehow, Adam ended up with the beeper, which came in handy when the Prince was kidnapped by Skeletor. Adam not only used it to attract Wolfbats, but Stratos as well.