Welcome Bear refers jokingly to the level 50-ish bears that roam the Western Plaguelands. The joke is that new Undead players, not knowing that WPL is a much higher level zone than the adjacent Tirisfal Glades where they begin, wander in from the relatively safe (and non-PvP) Tirisfal Glades and are horribly murdered by — usually — a decaying bear. Hence, the Welcome Bear. One variant is the Welcome Spider, another recurring Plaguelands mob.
A Variation on the Welcome Bear for Alliance players can be an Obsidian Elemental while exploring and accidently wandering into the Burning Steppes from the Redridge Mountains or even wandering into Duskwood from Elwynn Forest.