There is a Walking Tree unique in Act 1, but I don't think there can be elites. However, there are also mobs such as Fanatics (the exploding guys in Act 3) who have a low possibility of spawning elite versions.
Diablo has an achievement Champion's Collection for killing elite enemies of each type - that should give you a complete list of which mobs can have elites, and by extension identify those not on the list that can't be boss/elite. Afaik, there is no enemy which can only spawn as elite or boss versions but not both. Obviously, this is not true for uniques considering the tree encounter.
Some enemies that cannot be elite:
- Wall spitters
- Rock worms (there is a unique, Shaitan the Brood Mother)
@yx.: Mallet Lords can be unique as they're listed in the achievement, though they're one of the few that I haven't killed yet. I expect its a combination of rarity of the mob itself and since everyone prefers to farm the other acts to avoid off-screen charges & easier to find NV stacks.