Emphasizing unyielding nature:
* A rock in the river, refusing to budge even with the strongest current.
* A bull in a china shop, unstoppable and causing damage.
* A rusted bolt, refusing to turn even with the greatest force.
Emphasizing inflexibility:
* A tree rooted in concrete, unchanging despite external forces.
* A steel bar, inflexible and unyielding to any pressure.
* A rigid statue, refusing to bend even in the wind.
Emphasizing obstinacy:
* A mule with blinders, refusing to see any other path.
* A donkey with a stubborn streak, refusing to move even with the strongest tug.
* A stubborn ox, refusing to yield to the plowman's command.
The best metaphor to use will depend on the specific context and the desired effect.