Endermen will give buffs to other mobs around, and this also includes mobs in the nether (As of 1.10 they can spawn in the nether).

Buff List

Mob Buff / Ability
Skeleton (Any Type) All players around him will be affected by Weakness II
Shulker All players around won't be able to jump
Silverfish Have Resistance II
Endermite Have Resistance II
Creeper Have Regeneration II
Blaze Have Regeneration II
Spider (Any Type) Have Jump Boost I
Zombie (Any Type) Have Speed II
Pigman Zombie Have Speed II