An idea I had several months ago and placed in the {monster ideas} entry in ##crawl. Decided to give a go at coding it myself: it seems to work as desired. Based on a hound, though with just slightly better stats: it has two attacks, one plain, and one dealing sickness damage. It also has berserker rage as its "escape" spell.
Diff is against current trunk (r10689).
Diff against r10689.