In short, [it comes from its **CON modifier**.][1]

The numbers in brackets provide formula to roll for the monster's HP and it works just the same as with PCs. In this case the 15d8 comes from a medium (hence the d8; a small one would have a d6 and so on) monster on a 15th "level" (not really a normal level, just a concept used in building the stats) plus 15*[CON MOD]. So I would suspect that the monster has CON +5 (since 75 = 15*5).

You can then check that average HP is 15*4.5 + 75 = 142 (round down).


  [1]: http://media.wizards.com/2016/downloads/DND/DMBasicRulesV05.pdf#page=3