If I cast Icathian Rain with only an opponent's Pouty Poro (a deflect unit) on the board, do I have to pay six times the deflect cost?

Ruling: No, you do not have to pay all six deflect costs. Under the current rules, you can decline to pay costs for triggered abilities, and those triggers will not be put on the chain. Since Pouty Poro will die after the first trigger resolves, you only need to pay the deflect cost once (or decline the remaining triggers).

Sequence:

• Cast Icathian Rain and it goes on the chain
• When Icathian Rain resolves, it creates 6 reflexive triggered abilities
• For each trigger, you choose a target and decide whether to pay any incurred costs (like deflect)
• If you pay the deflect cost for the first trigger, it resolves and kills Pouty Poro
• The remaining triggers fizzle because their target is no longer valid
• You can decline to pay the deflect cost for the remaining triggers

Nuances:

• You might choose to pay for more triggers than needed if you anticipate the opponent using a pump spell to keep their unit alive
• You must still choose targets for each trigger before deciding whether to pay costs
• Players can react between each trigger resolving