#an ancient mandalorian clan Fett (which i hope he is from mando culture himself)

[cienie-isengardu]

Arla - Mandalorian name?

ARLA FETT was first presented as as unnamed Jango's sister in Jango Fett: Open Seasons comics mini-series

then properly introduced as Arla Fett in Republic Commando: Order 66 & Imperial Commando: 501st)

Jusik squatted down and resorted to a little mind influence, the most benign, to make her realize he meant her no harm. It was worth trying even if he was stretching their luck. Something told him he had to, and maybe it was simply that he'd walked by one inmate too many. "Ner gai Bard'ika," he said. "Tion gar gai? Gar aliit?" He'd told her his name was Bardan, and asked her name and her clan name. She stared at him. It was as if she didn't believe what she was seeing, or hearing. "Arla," she said. She glanced at the nurse as if the woman was eavesdropping. "Neyar gain Arla Vhett." It wasn't Mando'a, but it was close enough for any Mandalorian to understand. Jusik turned slowly, still squatting, to look at Skirata. The old sergeant's face was a study in suppressed shock. [...] "So who is it?" Vau asked plainly irritated. Mird watched the woman with head cocked tail slapping. "We'd better have a good reason for taking a psychotic killer with us tonight." "We have," said Jusik. "That's Arla Fett-Jango's missing sister." [RC:O66]

and who at some point became a member of the Mandalorian Death Watch (a group that killed her parents while hunting Jaster Mereel and True Mandalorians on Concord Dawn)

Jusik steeled himself to look. He was expecting worse. He wasn't sure if the dark brown mark was a tattoo, or a scar, or a branding mark, but he knew exactly what it was because he'd seen one only hours ago, or a version of it: the Death Watch emblem, the ragged winged W shape. It didn't surprise him. She'd been spoils of war as far as they were concerned, an animal to be used, and marked as their property. "A surgeon can remove that," Kina Ha said. "Would that help?" Arla pulled down her shirt again. "You don't get it. You can't guess because it's so bad." "Whatever it was, you were a child of fourteen, Walon tells me. When we're adult, we look back and judge our childhood actions by unfairly adult rules." Arla didn't turn around. "It's not a wound or a humiliation. It's a badge." "Explain." "After I was kidnapped, after it stopped being a nightmare, I stayed with them. I became one of them. I stayed. I could have run away. But I stayed." [IC:501st]

So whatever she was a direct descendant of Mandalorian Fett clan or she just "joined" Mandalorians, she can be count as one of them.

The newest sources material introduced two other Mandalorian women of the same name:

ARLA SHALE (source: SWTOR short story "Seeing Red", can be read here)

Ballag started picking fights and spilling blood to prove his strength. The latest and most daring trouble was with Clan Shale. They were an old and respected clan with the trophies and glory to prove it. The body on the screen deserved a better death with a better enemy. “What do you expect with Ballag running them?” Rass said. “He’s going to run Nerak into the ground if he keeps this up.” He scoffed. He didn’t like Ballag. Too stubborn to lead, too stupid to see it. “I’ve spoken to Arla Shale.” Jekiah said. “Fought with her during that raid on Darvannis. She’s solid but won’t let her people be treated like this. It won’t end without a lot more bodies. With Mandalore gone, we need to stand as one.”

and

“A duel?” Arla asked, her lined and scarred face betraying no emotion. Without a moment of consideration, she nodded to Jekiah. “As the Arbiter decides.” No hint of fear in her eyes, only steely certainty. Ballag hesitated. He stood a head taller than everyone. His eyes were an uncanny yellow and flicked from Jekiah to Arla, the tiniest hint of unease in his gaze. Arla was experienced- maybe not faster, maybe not stronger, but Ballag would have to be a blind to consider her an easy target. Arla arched an eyebrow at the pause.

ARLA HA'RANGIR (source: SWTOR)

The closer thing to ARLA in Mando'a dictonary I managed to find is the word MANDOKARLA which means to having the right traits / spirit for Mandalorian.

So I guess we can assume Arla is an old/ancient mandalorian name?