Foundations Commander

Legendary Creature — Human Warrior

Formidable — At the beginning of combat on your turn, if creatures you control have total power 8 or greater, target creature you control gains haste until end of turn. (It can attack and {T} no matter when it came under your control.)

Surrak, the Hunt Caller

Tales of Middle-earth Commander

Enchantment

Creatures you control have haste.
Formidable — Whenever you attack, if creatures you control have total power 8 or greater, target attacking creature you control fights up to one target creature you don't control.

Gimli's Reckless Might

Legendary Artifact — Vehicle

Living metal (During your turn, this Vehicle is also a creature.)
Haste
Formidable — Whenever Ultra Magnus attacks, attacking creatures you control gain indestructible until end of turn. If those creatures have total power 8 or greater, convert Ultra Magnus.

Ultra Magnus, Armored Carrier

Transformers

Legendary Artifact Creature — Robot

More Than Meets the Eye {2}{R}{G}{W} (You may cast this card converted for {2}{R}{G}{W}.)
Ward {2}
Whenever Ultra Magnus attacks, you may put an artifact creature card from your hand onto the battlefield tapped and attacking. If you do, convert Ultra Magnus at end of combat.

Ultra Magnus, Tactician

The List

Creature — Bear

Formidable — {1}{G}: Regenerate this creature. Activate only if creatures you control have total power 8 or greater.

Dragon-Scarred Bear

Creature — Elemental

Defender
Formidable — {G}: This creature can attack this turn as though it didn't have defender. Activate only if creatures you control have total power 8 or greater.

Glade Watcher

Dragons of Tarkir Promos

{R}, {T}, Sacrifice this creature: It deals 4 damage to target creature.
Formidable — {2}{R}: This creature has base power 8 until end of turn. Activate only if creatures you control have total power 8 or greater.

Crater Elemental

Dragons of Tarkir

Creature — Ogre Warrior

Formidable — {3}{R}{R}: Creatures you control gain menace until end of turn. Activate only if creatures you control have total power 8 or greater. (They can't be blocked except by two or more creatures.)

Atarka Pummeler

Dragon

Creature — Human Shaman

{R}: This creature gains flying until end of turn.
{1}{R}: This creature gets +1/+0 until end of turn.
Formidable — {4}{R}{R}: Create a 4/4 red Dragon creature token with flying. Activate only if creatures you control have total power 8 or greater.

Dragon Whisperer

Creature — Human Warrior

Trample
Formidable — Whenever this creature attacks, if creatures you control have total power 8 or greater, this creature gains first strike until end of turn.

Sabertooth Outrider

Formidable — {4}{G}: This creature gets +4/+4 until end of turn. Activate only if creatures you control have total power 8 or greater.

Atarka Beastbreaker

Vigilance
Formidable — {T}: Add {C}{C}{C}. Activate only if creatures you control have total power 8 or greater.

Circle of Elders

Creature — Cat Beast

Formidable — {2}{G}: Target creature blocks this creature this turn if able. Activate only if creatures you control have total power 8 or greater.

Lurking Arynx

{T}: Add two mana in any combination of colors. Spend this mana only to cast creature spells.
Formidable — {9}{G}{G}, {T}: Each player's life total becomes the number of creatures they control. Activate only if creatures you control have total power 8 or greater.

Shaman of Forgotten Ways

Creature — Elk

Formidable — Whenever this creature attacks, if creatures you control have total power 8 or greater, creatures you control gain trample until end of turn.