Enemies that either bypass or get out of melee combat WITHOUT being required to kill barracks or heroes, mostly when sufficiently far away from the exit, including enemies that don't engage in melee at all but NOT fliers. Candidates include: Troll Pathfinder, Gulaemon, Saurian Darter, Ghost, Hyena, Webspitter Spider, Twilight Heretic, Cavalier, Frozen Soul, and even supposedly harmless Sheep and Bunnies.
Razorboars are a question mark since they require barracks or heroes near their front but I'm not sure if they stop close enough to have to engage in combat. Plus, their charge damage doesn't kill the most durable heroes or barracks units. Dune Terrors and Quarry Workers do not count since melee makes them block their subterranean movements.
I would have made this a poll, but there are too many candidates. In fact, other candidates may include any I forgot to mention in the initial draft, again.