Khalni Hydra — ruling 1

For the purpose of determining the cost reduction, the number of green creatures you control is checked as you cast Khalni Hydra, before your last chance to activate mana abilities to pay for it. For example, if you control Wild Cantor (a red and green creature with the ability "Sacrifice Wild Cantor: Add one mana of any color.") as you cast Khalni Hydra, first the cost to cast Khalni Hydra is reduced by {G}, then you could sacrifice Wild Cantor for mana to help pay for it.

Khalni Hydra — ruling 2

Khalni Hydra's cost reduction effect doesn't change its mana cost or mana value.

Khalni Hydra — ruling 3

If an effect (such as the one from Lodestone Golem) imposes an additional generic mana cost to casting Khalni Hydra, the Hydra's ability will reduce it too. It'll reduce the amount of green mana you need to spend first, though.

Khalni Hydra : règles, fonctionnement et interactions (MTG)

Khalni Hydra is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

This spell costs less to cast for each green creature you control. Trample

How it works : Khalni Hydra

Original English text — Wizards of the Coast / Scryfall

#1June 15, 2010· Wizards of the Coast
For the purpose of determining the cost reduction, the number of green creatures you control is checked as you cast Khalni Hydra, before your last chance to activate mana abilities to pay for it. For example, if you control Wild Cantor (a red and green creature with the ability "Sacrifice Wild Cantor: Add one mana of any color.") as you cast Khalni Hydra, first the cost to cast Khalni Hydra is reduced by , then you could sacrifice Wild Cantor for mana to help pay for it.
No other card is named in this ruling.
#2June 15, 2010· Wizards of the Coast
Khalni Hydra's cost reduction effect doesn't change its mana cost or mana value.
No other card is named in this ruling.
#3June 15, 2010· Wizards of the Coast
If an effect (such as the one from Lodestone Golem) imposes an additional generic mana cost to casting Khalni Hydra, the Hydra's ability will reduce it too. It'll reduce the amount of green mana you need to spend first, though.
No other card is named in this ruling.