The Creature with Bloodthirst does not have a -1/-1 counter on it.
Steps
Activate
Cauldron of Souls by tapping it, giving the Creature with Bloodthirst persist until end of turn.
Activate
Megatog by sacrificing the Creature with Bloodthirst.
When the creature dies, its persist ability and
Dross Scorpion trigger.
Resolve the persist trigger, returning the creature from your graveyard to the battlefield, which enters the battlefield with at least one +1/+1 counter on it due to its bloodthirst ability.
Resolve the
Dross Scorpion trigger, untapping
Cauldron of Souls.
Resolve the
Megatog ability, giving it +3/+3 until end of turn.