Rin and Seri, Inseparable

Whenever you cast a Dog spell, create a 1/1 green Cat creature token.
Whenever you cast a Cat spell, create a 1/1 white Dog creature token.
{R}{G}{W}, {T}: Rin and Seri, Inseparable deals damage to any target equal to the number of Dogs you control. You gain life equal to the number of Cats you control.

Kaervek the Merciless

Whenever an opponent casts a spell, Kaervek the Merciless deals damage equal to that spell's mana value to any target.

Zuberi, Golden Feather

Flying
Other Griffin creatures get +1/+1.

Balthor the Stout

Other Barbarian creatures get +1/+1.
{R}: Another target Barbarian creature gets +1/+0 until end of turn.

Illuna, Apex of Wishes

Mutate {3}{R/G}{U}{U} (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Flying, trample
Whenever this creature mutates, exile cards from the top of your library until you exile a nonland permanent card. Put that card onto the battlefield or into your hand.