In the Werewolf: The Apocalypse roleplaying game (published by White Wolf Game Studios), metis is the term for the result of a union between two shifters of the same breed (two Garou, two Bastet, two Ananasi, et cetera).

Metis Biology

The product of such a union, at least in the case of the Garou1 is always another werewolf, rather than the usual one-in-ten probability that the child will exhibit the recessive lycanthropic trait2. Compare homid- and lupus-born Garou.

Metis are always born in Crinos form, just as homid Garou are always born in homid form and lupus Garou are always born in, well, lupus. They always carry at least one physical deformity of some kind, such as hairlessness, missing or deformed limbs, or damaged hearing. Metis are sterile; however, they are capable of the physical act of mating.

Metis Garou and Garou Social Structure

The easy source of new Garou cubs provided by the mechanism that produces metis would be a godsend for the Garou, whose numbers have been shrinking with every generation as Gaia decays more and more under the corrupting influence of the Wyrm. Unfortunately, through an unfortunate accident of culture combined with the Garou's overweening traditionalism and pride, metis are viewed as the very lowest of the low; they are the Unwashed, the Corrupted, the Outcast.

The reason for this is because of the congenital deformities of metis Garou. This has caused the Garou to believe that metis carry the mark of the Wyrm, and must be corrupt; some believe that they are spies planted by the Wyrm amongst Gaia's warriors; others simply believe that they are inferior.

For this reason, the Garou Litany forbids Garou to mate with Garou. Some of the other shifters have injunctions about mating, but by no means all. Even some Garou tribes adhere less stringently to this tenet of the Litany than others -- the Children of Gaia, Glasswalkers and Bone Gnawers are noted for ignoring this tenet, and are far more likely to be accepting of and friendly to any metis Garou that they may meet than members of the other tribes.

At the other extreme, metis Garou are killed at birth by some of the more hard-line Garou tribes (particularly the Shadow Lords, Silverfangs and Get of Fenris). This practice is falling out of vogue, however, as the Garou get rarer, and have begun to need every warrior for Gaia they can get their paws on.

...And some of the more liberal (and mystical) Theurges of the Garou Nation whisper that the metis may be Gaia's salvation...

Footnotes

1This writeup will deal with Metis from the Garou perspective, since the Garou are the most common shapeshifters in the World of Darkness and, well, the game is called Werewolf: The Apocalypse, not Bastet: The Spying or Nuwisha: The Tricking... and DEFINITELY not Corvax: The Shiny Thing...

2This is how a Progenitor or an unEnlightened scientist would view the Garou "gene". The Garou view of the (in)frequency of shifter births is rather more vague; the Garou don't really bother to try and explain it. One must remember that Garou are not entirely material creatures -- part of their being is literally spirit matter...