What did Freud intend as the referent of 'Das Es' (Latin: 'id')? Why didn't he name with the referent? Wikipedia states:


  The terms "id", "ego", and "super-ego" are not Freud's own. They are latinisations by his translator James Strachey. Freud himself wrote of "das Es",[5] "das Ich",[19] and "das Über-Ich"[28]—respectively, "the It", "the I", and "the Over-I" (or "I above"); thus to the German reader, Freud's original terms are more or less self-explanatory. [...]



What's the referent of the German pronoun 'Es'? 
Why didn't Freud name (the concept known as ⟨Es⟩) with the referent itself instead? I.e., wouldn't naming ⟨Es⟩ as 'Instinkt' to mean 'instinctual drive' make the term easier to understand and more self-explanatory?
Short answer
Freud din't come up with that terminology, he borrowed it from his contemporaries. 

Background
According to wikipedia Freud borrowed the term "das Es" from Georg Groddeck, a German physician to whose unconventional ideas Freud was much attracted (Groddeck's translators render the term in English also as "the It").

Nietzsche, who is thought to have influenced Freud also often used the German pronoun das Es. Freud’s English translator indeed used the Latin word 'id'.