Making appearances in the prequels are the familiars races such as:

Wookiees

The following info comes from the Hoth Chewbacca Figure File Card (courtesy of Sir Steve's):

Chewbacca, like all members of the Wookie species, was indigenous to the jungle planet of Kashyyyk. Honorable, intelligent, and friendly, though fierce and dangerous when angered. Wookiees were a proud species who once made contributions to the Galactic Republic. Wookiee senators represented Kashyyyk in the Galactic Senate. Their respectable position began to change however when the Imperials seized power and began enslaving the extraordinarily strong Wookies as laborers. In Chewbacca's case, he escaped slavery and death with the help of a young officer by the name of Han Solo, who destroyed his military career to save the innocent Wookiee. A life debt was established, and Chewbacca joined Han in his adventures through the galaxy. Later, the duo played significant roles in the Rebel victory over the Empire, and the Wookiee species was restored in its position of honor in the galaxy.

Twileks - These blue Twi'lek are played by Ninfan and Nishan Hinedes.

Tusken Raiders - One scene with the Tusken Raiders has them sniping from a cliff top, similar to ANH.

Quarren

Cantina Alien and Others - Seen above is the alien from the SE and another unknown alien in the streets of Mos Espa.

Gran (Ree-Yees)

Jawas

Hutts - It is known that Jabba will appear in the prequel.

Ithorians

Bith - There is also an alien band that plays at the pod race. Possibly Bith?

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NEW ALIENS

New aliens include this creature who's hand is being sculpted by an artist:

This alien appears in the Lynne's Diary Video #5:

These lizard/frog creatures also appear:

Below are pictured Pod Racer Ody Mandrell and an alien named Fanta.

UNKNOWN RACE - 12/16/97 - Shug Ninx gave us this tidbit in reference to this pic that recently emerged at Starwars.com.

''Check out the the update at the official site. If you look, closely at the unfinished alien on the table you'll notice what looks to be three tentacles around a mouth, the top of the head is flopped back behind it. It is the Sea Otter/Sea Lion type alien. It has three little beards, they kind of look like small pony tails. The alien on the table will be covered with what looks like soft silver/grey hair. It has no nose but instead has what kind of looks like a bisected blow hole.''