Umbreon

Umbreon boasts some of the best defensive stats of all Pokemon in the OU tier. Access to various types of support moves allows it to fit a niche role in a team—Mean Look to guarantee a free entry against an opposing Pokemon of your choice, Wish to heal its teammates, Heal Bell to remove status, Taunt to shut down opposing support Pokemon, and stat-dropping moves such as Fake Tears, Screech, and Charm to force switches. Its ability, Synchronize, also allows it to inflict a status ailment like burn, paralysis, poison, or badly poisoned that occurs due to a primary or secondary effect of a move. It is one of the few Pokemon that can use Wish + Protect + Pursuit in one set in order to be able to take out Psychic- and Ghost-types a lot more reliably, with Choice Scarf Tyranitar being the other Pokemon that is commonly used to to take out Psychic- and Ghost-types.

However, despite all of the support Umbreon can offer, it hardly ever sees any usage in OU. The reasons for this include Umbreon's passivity, since it has only base 65 Attack and an even lower base 60 Special Attack. Umbreon is also vulnerable to all forms of entry hazards; as well as prevalent sand, which not only inflicts chip damage but most importantly also halves the amount that Umbreon can heal through its only instant-healing move, Moonlight. Tyranitar, which also is one of the more common Pokemon to set sand, also gives Umbreon a lot of competition as one of the better Dark-types in the tier. Umbreon also used to see some usage when Baton Pass was around in DPP, but the move's ban explains Umbreon's lesser usage as well. All of this suggests to players that there may be better defensive options for them to run over Umbreon.