Depends on what you define a druid. Shamans, Witches, and Sorcerers of the Americas were considered shapechangers, particularly known for this ability. Merlin was usually considered a Wizard and was well known for shapechanging. Some have considered him a Druid for this and to tie him to early mythological/folkloric entities of Britain, but that reveals another problem: Both Myrddin Wyllt (Lailokan) and Taliesin, early mythic figures who inspired the character of Merlin, were considered Bards by the early medieval poets (notably, Emrys Wledig, a third influential figure on the character, was a Warlord and Seer, possibly emulatable in D&D by a College of Valor Bard, but you could also go with a number of divination-user classes that have martial bends). The identities of Bards and Druids and Wizards and Sorcerers and Warlocks and Clerics, really ALL SPELLCASTERS, are confused in mythic stories, because the boundaries as defined by D&D don't exist in real-life traditions. Some boundaries are made in some cultures, but these are the exceptions, not the rule.
The great thing about 5e is that you can take the class chassis and build your concept in many different ways. If you want to be a Green Knight character, you could be a Paladin (Oath of the Ancients), a Ranger, a Totem Warrior Barbarian, even a Nature Domain Cleric (due to its martial elements). If you want to be a Shaman, you might be a Nature Cleric, a Druid of the Land, a Druid of the Moon, an Archfey Patron Warlock of Book or Chains, you might be Divination Wizard, you might be a Wild Sorcerer. You might be a Bard, even.
So how does this relate to Warlords? A Warlord could be many different things, each with their own names, but approaching similar concepts.