(covers information from an alternate timeline)
In an alternate 2374, three Nihydron warships joined the USS Voyager in its attack on the Krenim weapon ship. B'Elanna Torres and Harry Kim were assigned to work with the Nihydron to implement temporal shielding aboard their ships. During the last conflict of the Year of Hell, two of the Nihydron vessels were erased from history by the weapon ship. This timeline was later negated by Kathryn Janeway. (VOY: "Year of Hell, Part II")
This species was only mentioned in dialogue.
The Nihydron reappeared in the 2016 novel A Pocket Full of Lies. The novel depicted a second iteration of Voyager's first contact with the species, since the events of the Year of Hell – where Voyager made first contact with them in 2374 – were erased from history. Seven of Nine identifies them as Species 1184 and they are described as having waxy flesh, small translucent eyes, wide noses, thin-lipped mouths and two large protrusions where a forehead would normally be.