A StealthBuck is a pre-war piece of technology that allow its user to render him/herself practically invisible.
During the War
StealthBucks were created by the Ministries of Technology and Arcane Sciences, from stealth technology recovered from the zebra, specifically their invisibility cloak. The spell used by the zebra was much better and able to keep a pony/zebra in a stealth field for far longer than a StealthBuck, which lasts roughly an hour. StealthBucks are attached to PipBucks, hence their name.
After the War
Like many other pieces of technology, StealthBucks survived the war and can be found in numerous locations. The spell within them has only one charge, making these devices single-use. Littlepip acquired one of these during her journey and made use of it on multiple occasions.
In Other Stories
Stealthbucks make numerous appearances in other stories.
The Crimson Hoof make extensive use of StealthBucks, employing specially trained scouts to use them and watch the roads.