Matango is a member of D'Hoffryn's Magic Council. He was an anthropomorphic mushroom. Apparently, he was very toxic and thus Rupert Giles prevented him from touching his sink with his spores.
After the restoration of magic, Buffy and the other Scoobies created a series of rules to write in the Vampyr book, where each member of the council would have a particular ability that they could use against those who would threaten Earth's balance.
Matango's ability is to be invulnerable to magical attacks, though not physical ones.
However, he was killed by D'Hoffryn when he burnt him to death with a flamethrower, as was planning on using the magic law for himself.