Name: Delibird
New Stats: 85 / 125 / 80 / 105 / 80 / 115
New Typing: Ice / Flying
Movepool Additions: Drain Punch, Grass Knot, Bulk Up, Baton Pass
New Abilites: Vital Spirit, Thick Fat, Refrigerate
Justification: Delibird is based on Santa Claus, so Thick Fat and Bulk Up were obvious choices, Drain Punch gives it somewhat reliable recovery, and Grass Knot fixes gaps in its special coverage
Niche in OU: A Refrigerate sweeper that doesn't take up a mega slot. It can also pass Bulk Up boosts to teammates.
Example Sets
Name: Delcatty
New Stats: 110 / 85 / 105 / 90 / 123 / 77
New Typing: Normal / Fairy
Movepool Additions: Hone Claws, Morning Sun, Dazzling Gleam, Refresh
New Abilities: Wonder Skin, Limber, Pixilate
Justification: Limber was added because cats are agile, Hone Claws, Morning Sun provides semi-reliable recovery, Pixilate gives Delcatty more Fairy-type moves to work with, and Dazzling Gleam allows Delcatty to have a special Fairy-type move without having to use Pixilate.
Niche in OU: Delcatty's improved stats and abilities allow it to fill a variety of roles on defensive teams, such as cleric support, stallbreaking, and setting up with Calm Mind.
Example Sets
Name: Ninjask
New Stats: 90 / 110 / 95 / 50 / 85 / 160
New Typing: Bug / Dark
Movepool Additions: Sucker Punch, Acrobatics, Storm Throw, Feint Attack
New Abilities: Speed Boost, Technician, Infiltrator
Justification: Ninjask's type was changed to Bug/Dark because it goes well with its "ninja bug" theme, and is a better defensive typing than Bug / Flying. Feint Attack and Sucker Punch are reliable STAB, Acrobatics and Technician were added because ninjas are usually acrobatic and fairly skilled, and Storm Throw fixes gaps in Ninjask's offensive coverage.
Niche in OU: A revenge killer that doesn't need a Scarf to do its job
Example Sets
Name: Unown
New Stats: 90 / 100 / 90 / 105 / 90 / 105
New Typing: Psychic
Movepool Additions: Ice Beam, Thunderbolt, Shadow Ball, Energy Ball, Dazzling Gleam, Dark Pulse, Earth Power, Psychic
New Abilities: Synchronize, Protean
Justification: Protean comes from Unown's knowledge of Hidden Power, which can be almost any type, Synchronize is the closest one can get to a reference to its Pokedex entries, Psychic gives Unown a reliable STAB move for Synchronize sets, and the other moves are there primarily to help it take advantage of Protean.
Niche in OU: Unown can hit most of the tier super-effectively with any combination of moves
Example Sets