Arcosians are a race presumably belonging to the Planet Trade Organization. They were named only in the FUNimation Dub of Dragon Ball Z.
Appearance
Their appearance has them wearing white cloaks and what resembles gas masks, as well as apparently being green.
They are most notably responsible for supplying the Saiyans with Battle Armors and other technology. In return for this technology, the Saiyans would conquer planets for the Arcosians.
Relation with Frieza's race
The Arcosians are never seen nor mentioned again after the flashback they appear in (in the episode "Goku's Ancestors"), but it has been suggested that they may actually represent Frieza's race.
This would be fitting given their relationship with the early Saiyans, as well as their small humanoid bodies concealed underneath their cloaks; as evidenced by Frieza's ancestor Chilled using a hooded cloak when encountering Bardock on Planet Plant in Dragon Ball: Episode of Bardock. Also, the one who destroyed Planet Vegeta was shown wearing a hooded cloak in an early flashback and we later learn that the one who destroyed the planet was Frieza himself.
In addition, it is to be noted that the name of the race partially derives from negative temperatures (in this case, Arctic), like the names of the members of Frieza's race. However this is an unintentional coincidence as this race was unnamed in the original Japanese broadcast.