Steer Clear

Steer Clear deals 2 damage to target attacking or blocking creature. Steer Clear deals 4 damage to that creature instead if you controlled a Mount as you cast this spell.

Wylie Duke, Atiin Hero

Vigilance
Whenever Wylie Duke becomes tapped, you gain 1 life and draw a card.

Throw from the Saddle

Target creature you control gets +1/+1 until end of turn. Put a +1/+1 counter on it instead if it's a Mount. Then it deals damage equal to its power to target creature you don't control.

Stubborn Burrowfiend

Whenever this creature becomes saddled for the first time each turn, mill two cards, then this creature gets +X/+X until end of turn, where X is the number of creature cards in your graveyard.
Saddle 2 (Tap any number of other creatures you control with total power 2 or more: This Mount becomes saddled until end of turn. Saddle only as a sorcery.)

Rambling Possum

Whenever this creature attacks while saddled, it gets +1/+2 until end of turn. Then you may return any number of creatures that saddled it this turn to their owner's hand.
Saddle 1 (Tap any number of other creatures you control with total power 1 or more: This Mount becomes saddled until end of turn. Saddle only as a sorcery.)

Fortune, Loyal Steed

When Fortune enters, scry 2.
Whenever Fortune attacks while saddled, at end of combat, exile it and up to one creature that saddled it this turn, then return those cards to the battlefield under their owner's control.
Saddle 1

Miriam, Herd Whisperer

During your turn, Mounts and Vehicles you control have hexproof.
Whenever a Mount or Vehicle you control attacks, put a +1/+1 counter on it.

Bucolic Ranch

{T}: Add {C}.
{T}: Add one mana of any color. Spend this mana only to cast a Mount spell.
{3}, {T}: Look at the top card of your library. If it's a Mount card, you may reveal it and put it into your hand. If you don't put it into your hand, you may put it on the bottom of your library.

Frontier Seeker

When this creature enters, look at the top five cards of your library. You may reveal a Mount creature card or a Plains card from among them and put it into your hand. Put the rest on the bottom of your library in a random order.

Giant Beaver

Vigilance
Whenever this creature attacks while saddled, put a +1/+1 counter on target creature that saddled it this turn.
Saddle 3 (Tap any number of other creatures you control with total power 3 or more: This Mount becomes saddled until end of turn. Saddle only as a sorcery.)

Bridled Bighorn

Vigilance
Whenever this creature attacks while saddled, create a 1/1 white Sheep creature token.
Saddle 2 (Tap any number of other creatures you control with total power 2 or more: This Mount becomes saddled until end of turn. Saddle only as a sorcery.)

Mobile Homestead

This Vehicle has haste as long as you control a Mount.
Whenever this Vehicle attacks, look at the top card of your library. If it's a land card, you may put it onto the battlefield tapped.
Crew 2 (Tap any number of creatures you control with total power 2 or more: This Vehicle becomes an artifact creature until end of turn.)

Forest

Bounding Felidar

Whenever this creature attacks while saddled, put a +1/+1 counter on each other creature you control. You gain 1 life for each of those creatures.
Saddle 2 (Tap any number of other creatures you control with total power 2 or more: This Mount becomes saddled until end of turn. Saddle only as a sorcery.)

Reach for the Sky

Flash
Enchant creature
Enchanted creature gets +3/+2 and has reach.
When this Aura is put into a graveyard from the battlefield, draw a card.

Creosote Heath

This land enters tapped.
When this land enters, it deals 1 damage to target opponent.
{T}: Add {G} or {W}.

Full Steam Ahead

Until end of turn, each creature you control gets +2/+2 and gains trample and "This creature can't be blocked by more than one creature."

Trained Arynx

Whenever this creature attacks while saddled, it gains first strike until end of turn. Scry 1. (Look at the top card of your library. You may put that card on the bottom.)
Saddle 2 (Tap any number of other creatures you control with total power 2 or more: This Mount becomes saddled until end of turn. Saddle only as a sorcery.)

Drover Grizzly

Whenever this creature attacks while saddled, creatures you control gain trample until end of turn.
Saddle 1 (Tap any number of other creatures you control with total power 1 or more: This Mount becomes saddled until end of turn. Saddle only as a sorcery.)

Seraphic Steed

First strike, lifelink
Whenever this creature attacks while saddled, create a 3/3 white Angel creature token with flying.
Saddle 4 (Tap any number of other creatures you control with total power 4 or more: This Mount becomes saddled until end of turn. Saddle only as a sorcery.)

Intrepid Stablemaster

Reach
{T}: Add {G}.
{T}: Add two mana of any one color. Spend this mana only to cast Mount or Vehicle spells.

Shepherd of the Clouds

Flying, vigilance
When this creature enters, return target permanent card with mana value 3 or less from your graveyard to your hand. Return that card to the battlefield instead if you control a Mount.

Ornery Tumblewagg

At the beginning of combat on your turn, put a +1/+1 counter on target creature.
Whenever this creature attacks while saddled, double the number of +1/+1 counters on target creature.
Saddle 2 (Tap any number of other creatures you control with total power 2 or more: This Mount becomes saddled until end of turn. Saddle only as a sorcery.)

Congregation Gryff

Flying, lifelink
Whenever this creature attacks while saddled, it gets +X/+X until end of turn, where X is the number of Mounts you control.
Saddle 3 (Tap any number of other creatures you control with total power 3 or more: This Mount becomes saddled until end of turn. Saddle only as a sorcery.)