(From Planescape: Monstrous Compendium Appdendix I - 1994):

Foo creatures are powerful extraplanar entities that can sometimes be convinced to serve or guard humans of good alignment.

Foo creatures have large, blunt heads with wide-set, bulging eyes. Their bodies appear somewhat leonine, with thick fur and large, padded feet. Their forelegs resemble those of lions, while their hind legs are more doglike.

Foo dogs are the most frequently encountered species of this creature. They have short, bush-tipped tails, long floppy ears, and broad noses. Their fur is typically golden or black, but also may be white, green, violet, or gray.

Each foo creature — dog or lion — speaks its own language as well as the language of other foo creatures. All foo creatures also speak the language of human or humanoid races with whom they have formed relationships.

Combat: Foo dogs bite and attack with their claws. When engaged in combat with opponents of evil alignment, the dogs attack as if they were 10 HD creatures. If the opponents are lawful evil, foo dogs gain a +1 bonus to their attack and damage rolls. Evil opponents who attack foo dogs suffer a -1 penalty to their attack rolls and inflicted damage.

Foo dogs can become invisible and ethereal at will. They can travel astrally and ethereally, both at will. In addition, their thunderous barking is 20% likely to create a gate that summons 1-6 additional foo dogs, provided the barking lasts 7 consecutive rounds or more.

Habitat/Society: Foo dogs lair in the Astral Plane, Ethereal Plane, Arborea, Ysgard, and others. Occasionally, characters of good alignment can convince foo dogs to join or guard them on the Prime Material Plane, especially if the person requesting assistance impresses the foo dog with kindness and benevolence, or if the cause appeals to the foo dog's sense of justice. However, a foo dog seldom voluntarily remains for long (a stay of more than a few weeks is rare).

Ecology: Foo creatures were once petitioners who served chaotic good powers, but they have earned proxy status and their present forms as a reward for excellent service. They don't remember much about their past lives, however. No new foo creatures have appeared in a long time, but then foo creatures live a long time. They, themselves, believe that a foo ages only when it's not performing good acts, so the rare aged-looking foos are sometimes snubbed by younger specimens. Eventually, very old ones move into isolated corners of the less-traveled planes. 

Foos can eat almost anything. They love gobbling gems and precious metals, especially platinum and silver, and an adventuring party must surrender a portion of recovered treasures to a foo creature if they hope to retain its company.

Foo Lions

Foo lions look like big foo dogs with longer tails and larger mouths. Shaggy manes encircle their necks. Sometimes the manes are entangled with peony flowers. A foo lion’s fur may be a variety of colors, but is most commonly black, orange, or gold.

Foo lions have the same outlook, personality, diet, and habitats as foo dogs. They also have the same abilities and attack techniques, but overmatched lions retreat and regroup faster than dogs do. Foo lions also favor sneak attacks, surprise, and siege tactics more than foo dogs; they are also more patient. Foo lions don't dislike foo dogs, but rarely associate with them. 

In combat with opponents of evil alignment, foo lions attack as 15-HD creatures. Evil opponents attack lions with the same penalties they suffer against foo dogs. If a foo lion roars for at least four consecutive rounds, it has a 20% chance of gating in 1-2 additional foo lions from another plane. 

Many good-aligned characters consider foo lions symbols of courage and strength. Emblems of foo lions are embroidered on court robes of military officials in the Beastlands, Mount Celestia, and in oriental cultures on the Prime Material. Stone statues of foo lions stand in front of many official buildings. Sometimes these statues and emblems are magically enchanted, making it possible to talk with a foo lion in its home plane. It won't always answer, though.