From Nighthawks Shadowrun Wiki
Slang for Yakuza members.
Furry bad tempered four legged things that spit!
Note: I think an element of eastern knowledge is called for here. Yaks do not spit, Lamas do. Yaks are gentle natured creatures with long hair, kind of like highland cattle.
Note 2: Also, contrary to some early claims, they do not migrate to Seattle to kill shadowrunners.