The alien merchants bound in vines.
As these two yellow-skinned traders were walking through the storeroom of the Fortress of Tawntoom, they were surprised to see a miniature R2-series astromech droid come strolling up to them. As they puzzled over the tiny droid, they were unexpectedly blasted by Mungo Baobab, who had been hiding behind a stack of storage crates. Due to the nature of Mungo's seed launcher, the aliens were not harmed, but instead became bound together by a rapidly growing vine that sprouted from the seed that had been fired. After stealing the merchant's cloaks to use as disguises, Mungo left the aliens bound with the vines as he, C-3PO and the miniature R2-D2 set out to infiltrate the fortress.
Since The Frozen Citadel aired in 1985, the species and home world of these aliens has remained unknown.