To give an alias to DynamoDB field with PynamoDB, use the attr_name parameter for the appropriate attribute constructor:

class Thread(Model):
class Meta:
    table_name = 'Thread'
forum_name = UnicodeAttribute(hash_key=True)
# This attribute will be called 'tn' in DynamoDB
thread_name = UnicodeAttribute(null=True, attr_name='tn')