The Botha were a reclusive, apparently humanoid species who inhabited a region of space in the Delta Quadrant. As the Botha protected their territory fiercely, there were lots of rumors about ships entering their space never to be seen again. Mithren had lost a number of vessels to them.
They were not seen as agressors, rather just as not wanting anyone crossing their borders. The Botha claimed to be concerned about vessels attacking their settlements. However, there was some belief that they did not have a legitimate claim to the space, and were merely vandals.
In 2372, the USS Voyager entered their space and attempted to negotiate passage. Almost the entire crew was neutralized in an elaborate series of hallucinations, apparently caused by a powerful psionic field generated by three Bothan ships. The hallucinations began with various elements from the target's memory, and gradually became more inclusive until the person entered a comatose state, trapped in a fantasy reality.
The Botha telepathy was defeated by Kes, whose telepathic abilities allowed her to "mirror" their attacks back onto the perpetrator (although she required aid from The Doctor – whose holographic nature also rendered him immune – to focus long enough to do so). This immunity allowed her to trigger a resonance burst from the warp core, which blocked the Botha's psionic field. However, both the Botha on Voyager and his ships were illusions, disappearing after informing Captain Kathryn Janeway that he had no reason for attacking them other than that he could. The Botha's true nature was never discovered. (VOY: "Persistence of Vision")