Band of the Black Earth

The creature is magically bound to a bulette trained to serve as it’s mount. While mounted on it’s bulette, the creature shares the bulette’s senses and can ride the bulette while it burrows. The bonded bulette obeys the creature’s commands. If it’s mount dies, the creature can train a new bulette to serve as its bonded mount, a process requiring a month. (Burrowshark, pp. 196, PotA)