Because fighting isn't combat damage, trample has no effect during the fight.
2019-07-12
If the target of Voracious Hydra's second mode isn't a legal target as the ability resolves, or if Voracious Hydra has left the battlefield, neither creature will deal or be dealt damage. The ability won't change to the first mode if the target is illegal.
Trample
This creature enters with X +1/+1 counters on it.
When this creature enters, choose one —
• Double the number of +1/+1 counters on this creature.
• This creature fights target creature you don't control.
