Hard Mode

Hard Mode bosses are variants of other bosses that have double HP and deal double damage, but have double rewards.

Hard Toriel - 200 HP and deals 8 damage. Rewards 160 Gold and 120 EXP.

Hard Papyrus - 500 HP and deals 8 damage. Rewards 360 Gold and 400 EXP.

Hard Mettaton Neo - 20000 HP and deals 500 damage. Rewards 12000 Gold and 12000 EXP.

Hard Sans
This guy is completely different.

Phase 1
He'll dodge 42 times (twice as much as regular sans) and deal 4 damage per hit.

Phase 2
In this phase he appears with his bad time eye showing. He'll dodge 21 times and do 8 damage per hit.

Rewards 16000 Gold and 38000 EXP.