Heavy units are a Class of unit that have the "Heavy" natural ability. Most of those seen thus far are considered the most powerful non-Commander units; presumably, since the only known Heavy 'abilities' are nerfs, the unit receives compensatory buffs that make it extremely useful in situations where their nerfs do not apply. Most heavy units are restricted from entering Tunnels, for example, and can't ride Mounts.
Heavy units may spontaneously pop in Ruins.
Known Heavy Units
Heavy Units may be unable to enter many terrain types besides Tunnels.
The Heavy Unit/Tunnel relationship is a bit of a conundrum. Supposedly, most Heavies are unable to enter tunnels. Tunnels are supposed to only be accessible to Light units. However, Sourmanders and Tchotchkes have been seen in the tunnels under Gobwin Knob, as have Spidews (which might or might not be heavies).
So: What proportion of Heavies are restricted tunnel access? Or were the GK caves so big that certain sections didn't count as "tunnels?" Could some heavies assist in the fight, but could not travel through the tunnels into the Dungeon? Or, is there a distinction between "heavies" meaning powerful non-knight, non-warlord units and units that have the "Heavy" special trait? Could "Heavy" just mean exactly that, a weight/size consideration independent of being a "heavy" (ie a tough guy, a ringer, a heavy hitter)?