Acorna's Quest by Anne McCaffrey

This SF team's earlier novel, Acorna, introduced the alien girl Acorna, an orphan rescued and reared by space-faring humans. Though generally human-shaped, she shares some features with the unicorn of myth--notably, a forehead horn that can heal and purify. Now Acorna blasts off with human friends in search of her own race, who shortly turn up looking for her. These gentle, horned nonhumans are fleeing other aliens who are wanton murderers and torturers that resemble huge cockroaches. Meanwhile, a starship full of dispossessed human miners picks up a scientist whose devastating system of planetary weather control inspires some on-board criminals to stage a coup and use the weather machine to blackmail whole planets; Acorna stumbles into this conflict. Meanwhile, a spacefleet of killer roaches is on its way.