Scion of Stygia

Flash
Cone of Cold — When this creature enters, choose target creature an opponent controls, then roll a d20.
1—9 | Tap that creature.
10—20 | Tap that creature. It doesn't untap during its controller's next untap step.

Grey Havens Navigator

Ambush Viper

Koilos Roc

Flash
Flying
When this creature enters, create a tapped Powerstone token. (It's an artifact with "{T}: Add {C}. This mana can't be spent to cast a nonartifact spell.")

Saiba Cryptomancer

Flash
Backup 1 (When this creature enters, put a +1/+1 counter on target creature. If that's another creature, it gains the following ability until end of turn.)
Hexproof

Crookclaw Transmuter

Flash
Flying
When this creature enters, switch target creature's power and toughness until end of turn.

Chrome Prowler

Flash
When this creature enters, tap target creature an opponent controls.

Shambleshark

Flash (You may cast this spell any time you could cast an instant.)
Evolve (Whenever a creature you control enters, if that creature has greater power or toughness than this creature, put a +1/+1 counter on this creature.)

Winged Coatl

Flash (You may cast this spell any time you could cast an instant.)
Flying
Deathtouch (Any amount of damage this deals to a creature is enough to destroy it.)

Haze Frog

Flash (You may cast this spell any time you could cast an instant.)
When this creature enters, prevent all combat damage that other creatures would deal this turn.

Pestermite

Flash
Flying
When this creature enters, you may tap or untap target permanent.

Frilled Mystic

Flash
When this creature enters, you may counter target spell.

Faerie Invaders

Flash (You may cast this spell any time you could cast an instant.)
Flying

Psionic Snoop

Flash
When this creature enters, it connives. (Draw a card, then discard a card. If you discarded a nonland card, put a +1/+1 counter on this creature.)

Breaching Hippocamp

Flash (You may cast this spell any time you could cast an instant.)
When this creature enters, untap another target creature you control.

Ambush Wolf

Flash (You may cast this spell any time you could cast an instant.)
When this creature enters, exile up to one target card from a graveyard.