I nickname most of my legendarys by taking a part of their name and doubling it, changing it around, reverse it [like with Ekans or Arbok], what theyre based on or base it on another pokemons name
Articuno - Uno, Tic Tic, Artic, Onucitra
Zapdos - Dos, Zap Zap, Sodpaz
Moltres - Tres, Molten, Phoenix, Sertlom
Mewtwo - Mew Mew Two, Owtwem
Ho-oh - Oh-ho
Lugia - Lu Lu, Aigul
Celebi - Celebrat[e/ion], Le Le, Bi Bi, Ibelec
Raikou - Kou Kou, Uokiar
Entei - Tei Tei, Ietne
Suicune - Ku Ku, Sui Sui, Cu Cu, Enucius
Regis - Rocky, Stealix, Icy, eciger, kcoriger, leetsiger
Latias - La La, Tia, Satial
Latios - Lo Lo, Tio, Sotial
Groudon - Grou Grou, Growl, Noduorg
Kyogre - Ky Ky, Gre Gre, Ergoyk
Rayquaza - Ray Quay, Azauqyar
Jirachi - Jirachirisu, Evil Eye, Ihcarij
Deoxys - De De, Syxoed
Uxie - Pixie, Eixu
Mesprit - Sprite, Tirpsem
Azelf - Elf, Fleza
Heatran - Heat, Hot, Volcano, Nartaeh
Dialga - Ga Ga, Aglaid
Regigigas - Gi Gi Ga, Sagigiger
Giratina - Ra Ra, Anitarig
Cresselia - Cresent, Ailesserc
Phione - Phio Phio, Enoihp
Manaphy - Mana Mana, Phee Phee, Yhpanam
Darkrai- Rai Rai, Nightmare, Dreamer, Iarkrad
Shaymin- Shay Shay, Mi Mi, Nimyahs
Arceus - Ce Ce, Zeus, Suecra
And my personal favourite
Mew - Mew Mew, Wem
I only really name legendarys but I can help anyone with nicknames... If its okay with the pplz that started this thread [=