Does Rengar Pridestalker effect proc when playing token units?

Yes, the legend ability of Rengar, Pridestalker triggers when you play a token unit.

According to FAQ #3881, playing a recruit unit token counts as playing a unit. Additionally, while tokens are not considered cards, they are nonetheless "played" when created by abilities (as confirmed by Rule 350.2 and FAQ #9271). Because the legend ability of Rengar, Pridestalker triggers "When you play a unit," and token units are units that are played, the trigger condition is met.

As established in FAQ #9769, you may choose the unit you just played (in this case, the token) as the target for the +1 Might bonus if you wish.