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Mule
Animal Hybrid
Understand that 'mule' refers to a specific animal hybrid and not just any mixed-breed animal. 
Workload Carrier
'Mule' highlights the animal's historic role in hauling burdens, making it a metaphor for bearing heavy loads. 
Stubborn Reputation
Mules are famously stubborn, so 'stubborn as a mule' implies great obstinacy. 