A nickname is given to a Denjuu by a T-Fanger.
In Telefang 1[]
When befriending a Denjuu, the T-Fanger has the option to nickname a Denjuu. It is entered using the D-Shot. Nicknames are restricted to six characters (kana or numbers). While nearly all unevolved Denjuu have no more than 6 kana, there are a few Denjuu that can be found in the wild and are more than 6 kana long. The most notable is Firekokko (ファイアーコッコ), where its default nickname will shorten to ファイアーコ. Denjuu's nicknames do not change at all upon evolution, even if they were not nicknamed.
Denjuu that are found in events, like the starter Denjuu Crypto and Fungus, among others, cannot be nicknamed.
In the bootlegs Pokémon Diamond and Jade, nicknaming is impossible, either because of a glitch or because the bootleggers could not make the phone type out Latin characters. As a result, Denjuu are auto-nicknamed to the first six characters of their bootleg name, with the exception of the Lampgera received in Iris, which has the glitchy nickname "o". Also, due to numerous bootleg Denjuu names being longer than their original Japanese names, the 6-character limit is far more prominent (for example, Crypto, called Kuribute in the bootleg, will be called "Kuribu").
In Telefang 2[]
Nicknaming in Telefang 2 is similar to Telefang 1, but the 6-character limit was removed and is now 8 characters (the maximum possible length for a Denjuu name). Also, it is possible to input Latin characters (albeit only in capital letters) instead of just kana or numbers.