Nós não sabemos; ele é chamado de "cavalariço".

Em toda a cena, ele é chamado de "cavalariço" ou "o menino", e nenhum outro nome é dado para ele:

Needle went through his leather jerkin and the white flesh of his belly and came out between his shoulder blades. The boy dropped the pitchfork and made a soft noise, something between a gasp and a sigh.
[...]
Blood had gushed from the boy's mouth as he collapsed, and more was seeping from the slit in his belly, pooling beneath his body. His palms were cut where he'd grabbed at the blade. She backed away slowly, Needle red in her hand.
[...] It would be so easy to hide here, as she had hidden from the wizard and the man with the forked beard. She could almost see the stableboy standing against the wall, his hands curled into claws with the blood still dripping from the deep gashes in his palms where Needle had cut him.
[...]
The memory made Arya smile, and after that the darkness held no more terrors for her. The stableboy was dead, she'd killed him, and if he jumped out at her she'd kill him again. She was going home. Everything would be better once she was home again, safe behind Winterfell's grey granite walls.
_{A Song of Ice and Fire, book 1: A Game of Thrones - Arya IV}

Até onde eu sei, ele nunca mais falou depois disso, e nunca sabemos o nome dele.

No programa, ela apenas o apunhala acidentalmente quando está tentando tirar o Needle do carrinho, e aprendemos ainda menos sobre ele.

Veja também o artigo ' stableboy ' no wiki de Game of Thrones (que também não sabe o nome dele).

Game of Thrones

De acordo com o show canon, também não sabemos. IMDB refere-se a ele como "Red Keep Stableboy", de modo que é provavelmente o nome oficial dos personagens .

A wikia sobre ele também se refere a ele como "Stableboy".

Westeros.org também se refere ao personagem como apenas "Stableboy".

Arya Stark: Needle.
A stable boy appears.
Stable Boy: There she is.
Arya Stark: What do you want?
Stable Boy: I want you, wolf girl. Come here.
Arya Stark: Leave me be. My father's a lord. He'll reward you.
Stable Boy: She'll reward me, the Queen!
Arya Stark: Stay away!
Accidentally, she turns around sliding Needle through the stable boys belly. Her first kill is a fact.
Shook as she is, she runs off.
_{Game of Thrones, Season 1 Episode 8, “The Pointy End”}

Uma canção de gelo e fogo

Mais uma vez por cânone do livro não sabemos, quando o cavalariço tem a chance de se apresentar ele declina.

"There she is," a voice hissed close behind her.
Startled, Arya whirled. A stableboy stood behind her, a smirk on his face, his filthy white undertunic peeking out from beneath a soiled jerkin. His boots were covered with manure, and he had a pitchfork in one hand. "Who are you?" she asked.
"She don't know me," he said, "but I knows her, oh, yes. The wolf girl."
_{A Game of Thrones, Arya IV}

Toda vez que ela se lembra do evento mais tarde, ela se refere a ele como "Stableboy" também, por exemplo.

Arya was remembering the stableboy at King's Landing. After him there'd been that guard whose throat she cut at Harrenhal, and Ser Amory's men at that holdfast by the lake. She didn't know if Weese and Chiswyck counted, or the ones who'd died on account of the weasel soup . . . all of a sudden, she felt very sad. "My father was called Ned too," she said.
_{A Storm of Swords, Arya VIII}