The Bothan starships were illusory vessels created by a telepathic member of the Botha during his encounter with USS Voyager in 2372. Only one of the ships actually existed, the other two being figments of the crew's imagination caused by the Bothan's psionic field. (VOY: "Persistence of Vision")
Background information Edit
The appearance of what was otherwise seen as a Kazon raider may be an example of Neelix's comment stating: "There are lots of rumors about ships entering Bothan space never to be heard from again." In addition, "my sources on Mithren say that they have lost a number of vessels," also suggests that one of these designs formerly belonged to the Mithren.