Bruse Tarl, Roving Rancher

Oxen you control have double strike.
Whenever Bruse Tarl, Roving Rancher enters or attacks, exile the top card of your library. If it's a land card, create a 2/2 white Ox creature token. Otherwise, you may cast it until the end of your next turn.

Purphoros's Emissary

Bestow {6}{R} (If you cast this card for its bestow cost, it's an Aura spell with enchant creature. It becomes a creature again if it's not attached to a creature.)
Menace (This creature can't be blocked except by two or more creatures.)
Enchanted creature gets +3/+3 and has menace.

Makindi Ox

Landfall — Whenever a land you control enters, tap target creature an opponent controls.

Earth-Origin Yak

When Earth-Origin Yak enters, creatures you control get +1/+1 until end of turn.

Holy Cow

Flash
Flying
When Holy Cow enters, you gain 2 life and scry 1. (Look at the top card of your library. You may put that card on the bottom.)