There are six types of Beyonder that are commonly encountered. Other than these types, what or who ever comes through a Gate is totally  random.

Shivans

Shivans are a multiform race that inhabits several planets connected by teleportation Gates. Sometimes these Gates go amiss, throwing travellers to other dimensions. A group of Shivans, called Scientists-Explorers, devoted themselves to exploring other dimensions. Shivans are roughly elfish. They may have two or four arms, two or three eyes and one or two faces on a single head. Females have four breasts and males have a pouch. Their names reflect their configurations: number of arms/color/Greek letter reflecting eye and face configuration/family name/ place name. Example:  4 Green Delta Anka of the N'Thala Ankas.

One notable Shivan, 4 Blue Beta Sila of the A'gala Silas, has become a Neophyte Savant, in hopes of learning the Secrets of the Worldstree and returning her people home.

Cultural Contact Units

CCU's come from a static machine civilization. This omniprocessor sends CCU's through Gates, knowing that one day they might return, bearing knowledge of other worlds. CCU's look like golden metallic balls, covered with plates and blinking lights. They can extend multipurpose arms at will. Their major advantage is that they are very likable and get along with everyone. They have Cultural Adaptibility, Language Talent, Savoir Fair (universal) and the ability to sense Gates, to which they are drawn.
 

Plasma Beings

Naming pattern:  Noble gas+ Numeric modified+halogen+binary number. Thus Neon diFlouride 00101 and Xenon tetraChloride 1101

Plasma beings appear as sparkling glowing clouds. They are soundless, but communicate through pulses of light and color. Apparently their home dimension collapsed and all Plasma Being are now transdimensional wanders.

Ishtarans

Ishtarans are elfish females that claim to be from a world called Sol II or Venus, from the nation of Istar Terra.  They use ultratech machinery, including electrolasers and force shields. They are not immortal, but are parthenogenic and transmit their memories to their offspring. Apparently their society just discovered D-Gates, and is in the process of sending out exploring teams.

RedSun Saurians

The Saurians are an acient race of Reptile Men long in contact with the Astologers of Silverymoon. Their homeworld has one star and one moon, thus though they are very advanced, they only recently discovered Gates. Thier Sun is dying, as is their cilivization. With the technical advice of the Astrologers. the Saurians split their huge Moon into 12 new moons, each with a different composition and orbit. Thanks to this, they can now open many Gates and are evacuating their world. Saurians are calm, staid and stoic, but eternally grateful to the Celenes.

Abyssians/Demons

Demons can only be summoned from Gates that open on the roots of the Worldstree. They are the allies and servants of the Damethear. Demon have no uniform appearance, but often look like a montage of animals and monsters. They are insane and evil, always.