The silver

The ​silver ​is a mare given to Daenerys Targaryen by Khal Drogo as a wedding gift.

Season 1
At their wedding, Drogo gives Daenerys the silver as a gift. Daenerys declares that she is beautiful and asks Ser Jorah Mormont what the word for "thank you" is in Dothraki. Jorah replies that there is no such word. Dany rides the silver as she and Drogo depart to consummate their marraige

In pain after being raped by Drogo, Daenerys needs to be helped down from the silver by her handmaidens.

Daenerys rides the silver through the Dothraki Sea and on to Vaes Dothrak.

After a wineseller is caught attempting to poison Daenerys, he is punished by being stripped and bound with a rope to the silver's saddle and is pulled along the road as the khalasar makes it's way to Lhazar.

Daenerys leaps from the silver to help Drogo when he falls from his horse, a wound he received in his duel with Mago having festered due to treachery of Mirri Maz Duur

Season 2
The silver dies of starvation in the Red Waste. Daenerys takes her death very hard, as she was the first gift Drogo gave her.

In the books
In the A Song of Ice and Fire novels, Drogo gives Daenerys the silver as a wedding gift. However, she does not die in the Red Waste but survives on a diet of devilgrass.