Is Rengar considered Mighty when played into a battlefield that you are attacking, for effects like Volibear that trigger when you play a Mighty unit?

Ruling: No, Rengar is not considered Mighty when played. He enters play at 3 power without the attacker designation, and only gains the attacker designation (and becomes 5 power) during the cleanup triggered by his entering the board.

Sequence:

• Rengar enters play at 3 power without attacker designation
• His play effect triggers (relevant for effects like Volibear)
• Cleanup is triggered by his entering the board
• During cleanup, he gains the attacker designation and becomes 5 power

Nuances:

• Because he enters at 3 power, he would trigger effects that care about non-Mighty units entering play (like Fiora in sealed)
• This differs from a unit with a passive ability like base 4:Might - such a unit would have the passive take effect as soon as it enters (when it's resolving) and would be Mighty for effects like Volibear