These three aliens appeared in separate episodes during the third season. It is later revealed that they were working together on a complex scheme of Zs'Skayr's design. It is never explained how this came about, since Zs'Skayr was apparently fully incapacitated throughout the season but as Zs'Skayr was the High Ecto-Lord of Anur Phaetos he would have been able to gather minions easily, another possibility is that the real Zs'Skayr that wasn't in the Omnitrix was the one who sent these three. An amusing fact is that Zs'Skayr and his minions are all based on horror monsters (Frankensteins' monster, a mummy, a werewolf, and a ghost), it is likely that all aliens from the Anur Belt are the basis for modern Horror Monsters.

Doctor Vicktor

Is a Transylian that resembles Frankenstein's monster, Doctor Vicktor works at NASA, disguised as a human scientist as a means to hijack a space shuttle. He is incredibly intelligent, and working for Zs'Skayr; his immediate plan was to resurrect his master, whose ultimate plan was to prevent the sun's light from reaching the earth using various satellites (therefore meaning that Zs'Skayr would be at full power all of the time). He serves as the unofficial leader of the three horror-themed aliens. The purple lightning that the previous two horror aliens appeared in was generated by a teleportation machine that he uses. His name is derived from the name of Victor Frankenstein from the Mary Shelley novel. The natural color of lightning that Transylians produce are green rather than purple.

His first appearance is in "The Return" where he succeeds in reviving Ghostfreak. In the next episode "Be Afraid of the Dark" Ben fights Vicktor and Ghostfreak on Earth. Ben accidentally gains use of Doctor Vicktor's form when Vicktor finds Ben and grabs him by the wrist, unintentionally adding his DNA to the Omnitrix. Vicktor essentially defeats himself near the end of the episode. In an attempt to teleport Ben to The Himalayas, he and the mummy alien are tossed into the portal he created.
Doctor Vicktor has only made two appearances in the Ben 10 Series.

Unnamed Mummy Alien

This mummy alien is a Thep Khufan that appeared in "Underwraps". This alien mummy was sent by Doctor Vicktor to find a dangerous element called corrodium that crashed on Earth millions of years ago. Grandpa Max explains that the crystals are a potent but unstable power source, and the radiation emitted by the crystals causes almost instant mutation in humans and other Earth-born creatures.

Todd Maplewood a boy Ben and Gwen met, said it arrived in purple lightning, which is the same way the Yenaldooshi arrived in "Benwolf" Max states there may be a connection between the Yenaldooshi and the mummy alien, which is proven in "The Return".

During Ben's battle with the creature, its DNA is absorbed by the Omnitrix, giving Ben access to the form. The mummy alien is frozen solid by Upgrade and buried, along with its corrodium, under a thick bed of concrete. It was released by the Yenaldooshi in "The Return" and assists in Dr. Vicktor's plan to plunge the earth into eternal darkness. When Ben is fighting Ghostfreak, Doctor Victor, and the mummy alien in "Be Afraid of the Dark" Doctor Vicktor inadvertently sends both himself and the Mummy Alien to an unspecified location through an ill-timed use of his teleportation device. The mummy alien has only made two appearances in the Ben 10 Series.

The Yenaldooshi

The Yenaldooshi is a Loboan (a species of werewolf-like aliens) sent by Doctor Vicktor to steal communication equipment from a reservation that Ben, Gwen, and Grandpa Max were visiting in "Benwolf" Ben first fights against the alien as Wildvine. When Ben is transformed back to his normal self by the Omnitrix, the alien werewolf slashes the Omnitrix, causing the device to download his DNA (signified by its face turning yellow) and gradually change Ben into the form.

In their second battle, the Yenaldooshi appears to be killed by a cave-in, though it is not actually shown dead. The device that it was building is shown coming online at episode's end, its purpose left unrevealed. The Yenaldooshi is revealed to have survived in "The Return", and its device is used as part of Ghostfreak's plan to block out the sun. In the next episode "Be Afraid of the Dark", the Yenaldooshi's transmitter explodes when Grandpa Max rams a space shuttle into Ghostfreak's Corrodium Projector, destroying it. It is unknown if the Yenaldooshi survived the explosion of the transmitter.
The Yenaldooshi has only made three appearances in the Ben 10 Series.