create alias pokemon_ultra_sm -> pokemon_usum
This is @Provence 's idea from topic #15021.
I thought pokemon_ultra_sm was good, but pokemon_usum is shorter and seems to be more used on the internet. It has more Google results ("pokemon ultra sm" = 40,100 results; "pokemon usum" = 218,000).
Apparently Pixiv uses the tags ポケモンUSM and ポケモンUSUM, but the "USM" has only two pages; "USUM" has lots of pages.
https://www.pixiv.net/search.php?s_mode=s_tag_full&word=ポケモンUSM
https://www.pixiv.net/search.php?s_mode=s_tag_full&word=ポケモンUSUM
The tag pokemon_usum also seems consistent with tags like pokemon_hgss (Pokémon HeartGold and SoulSilver) and
pokemon_oras (Pokémon Omega Ruby and Alpha Sapphire), in which all words other than "Pokémon" are part of the abbreviation.
EDIT: The tag alias pokemon_ultra_sm -> pokemon_usum (forum #143482) has been rejected by @Hillside_Moose.
Updated by DanbooruBot