Los Angeles County deputies allege Jared Foster made the spear by taping a large kitchen knife to a broomstick, the Los Angeles Times reported Thursday.
Foster, of Castaic, north of Los Angeles, is accused of using the spear, and possibly an arrow to kill the cat.
The dog was killed with the spear, deputies said, before Foster burned its body in the back yard.
Foster's brother found "a large amount of blood inside the home" and fled the home, deputies said. The brother then called deputies, telling them Foster was acting "irrationally and aggressive."