Can Rengar's Legend effect be interrupted by a Reaction before the unit gains the bonus Might?

Yes. Rengar, Pridestalker's legend ability is a triggered ability that goes on the Chain, so it doesn't apply the +1 Might until it resolves — after both players get Reaction windows (FAQ #11377, Rule 383.3).

The target is locked when the trigger finalizes (Rule 352.4.b), but the effect itself can still be stopped before resolution. If your opponent reacts by Killing, recalling, or otherwise removing the chosen unit from the board, the buff fizzles and the unit never gains the Might. The trigger itself can also be countered by effects that counter an ability that "chooses" that unit.

One nuance: because the target is chosen at finalization, a unit your opponent reveals Hidden in response can't be redirected onto the trigger — it's already locked in.