There is two things here which require an explanation:
First, what or who does es refer to? As others mentioned, this is referring to Geschäftsdiener. There might be a typo involved, and maybe it is supposed to be Er instead of Es.
Second, what does it mean to be a Kreatur des Chefs? In current German, one would rather say Er war ein Geschöpf des Chefs. The phrase expresses that the Geschäftsdiener has been made, created by the boss. Hence, it expresses that he is totally dependent and obedient to the boss. The word Kreatur is originating in Latin (creare, "erschaffen, wählen, hervorbringen", "to make, to select, to create"), and means Geschöpf ("the one that is made"). In German it usually carries negative connotations such as erbärmlich ("deplorable", "pathetic"), indicating low status. More strongly so than in English, it carries a meaning of passiveness and dependency.
It might not even be a special use of the term by Kafka, but might just have been common at his time, when knowledge of Latin was assumed as a sign of higher education. To me, it transports a rather formal, stiff style of speaking.