A Yeoman is not a type of being; it's a rank rate (thanks Monty129.) Star Trek simply inherited the title, much like the other military titles it uses. (Captain, Ensign, Admiral, First Officer, etc.)
Stealing from Memory Alpha :
Yeoman was a Starfleet title with administrative and clerical duties. Starfleet used different rates of yeoman including "yeoman third class" as seen with Tina Lawton. (TOS: "Charlie X") Some yeoman, such as Janice Rand, were enlisted personnel during their time in this position. However, a yeoman could hold the rank of an officer, as seen with Martha Landon (TOS: "The Apple") and an unnamed yeoman assigned to the USS Enterprise-A, who wore an officer's uniform with the rank of lieutenant junior grade. (Star Trek V: The Final Frontier)