Who is Mithra?

Before Zarathushtra, the ancient Persians worshipped many gods and goddesses. One of them was Mithra, who was the god of the sun, contract, friendship and justice.
Zarathushtra believed in only one God, Ahura Mazda. So, he rejected Mithra. As a result, Mithra is not mentioned in the Gathas. However, in the Avesta, he appears in the Mehr Yasht.
There he appears as “the lord of wide pastures who has a thousand ears and ten thousand eyes.” Some Achaemenian kings like Artaxerxes II (404 BCE) worshiped Mithra along with Ahura Mazda.
The popularity of Mithra eventually spread to Rome and after 136 AD many Roman soldiers and high-rank officials became the follower of Mithra. By the fourth century, however, the Mithraic cult had collapsed because Emperor Constantine converted to Christianity.
Zarathushtra believed in only one God, Ahura Mazda. So, he rejected Mithra. As a result, Mithra is not mentioned in the Gathas. However, in the Avesta, he appears in the Mehr Yasht.
There he appears as “the lord of wide pastures who has a thousand ears and ten thousand eyes.” Some Achaemenian kings like Artaxerxes II (404 BCE) worshiped Mithra along with Ahura Mazda.
The popularity of Mithra eventually spread to Rome and after 136 AD many Roman soldiers and high-rank officials became the follower of Mithra. By the fourth century, however, the Mithraic cult had collapsed because Emperor Constantine converted to Christianity.

Here is a simplified Avesta prayer from Khorshid Niyayesh (Prayer to the Sun):
In the name of God. I praise the creator Ormazd, the radiant, glorious, all-knowing, lord of lords, watchful, creator of the universe, powerful, wise, forgiver, merciful and loving.
Homage to you, O Ahura Mazda! Homage to the Sun!
I praise good thoughts, good words, and good deeds! I accept all good thoughts, good words, and good deeds. I renounce all evil thoughts, evil words, and evil deeds.
We praise Mithra
The lord of all countries,
Whom Ahura Mazda created the most glorious.
So may there come to us for aid
Both Mithra and Ahura, the two exalted ones.
In the name of God. I praise the creator Ormazd, the radiant, glorious, all-knowing, lord of lords, watchful, creator of the universe, powerful, wise, forgiver, merciful and loving.
Homage to you, O Ahura Mazda! Homage to the Sun!
I praise good thoughts, good words, and good deeds! I accept all good thoughts, good words, and good deeds. I renounce all evil thoughts, evil words, and evil deeds.
We praise Mithra
The lord of all countries,
Whom Ahura Mazda created the most glorious.
So may there come to us for aid
Both Mithra and Ahura, the two exalted ones.

Homage= respect and honor
Renounce= reject
Exalted= glorious, noble, grand
Renounce= reject
Exalted= glorious, noble, grand