When you play Riptide Rex, do you have to target a unit if there is one at a battlefield? If there is a unit with Deflect, can you choose to not do the ability?

Ruling: When Riptide Rex's triggered ability goes on the chain, you must choose a target unit at a battlefield if one exists. However, if that unit has Deflect, you may decline to pay the Deflect cost, which causes the triggered ability to fail to finalize on the chain and be removed without resolving.

Sequence:

• Play Riptide Rex and pay its cost; it enters the battlefield
• Riptide Rex's "When you play me" ability triggers and attempts to be placed on the chain as a Pending Item
• You must choose an enemy unit at a battlefield as the target
• If that unit has Deflect, the ability incurs a mandatory Deflect cost
• You may choose to decline to pay the Deflect cost
• If you decline, the ability is removed from the chain and never resolves (the 6 damage is not dealt)

Nuances:

• Declining to pay the Deflect cost is not considered countering the ability; it simply fails to finalize
• The ability never becomes a Chain Item if you decline the cost; it is removed while still a Pending Item