Bonded Horncrest can't attack or block alone.
9/29/2017 If you control more than one Bonded Horncrest, they can attack or block together, even if no other creatures attack or block.
9/29/2017 Although Bonded Horncrest can’t attack alone, other attacking creatures don’t have to attack the same player or planeswalker. For example, Bonded Horncrest could attack an opponent and another creature could attack a planeswalker that opponent controls. Similarly, other blocking creatures don’t have to block the same creature that Bonded Horncrest blocks.
9/29/2017 Once Bonded Horncrest has attacked or blocked, it will remain in combat even if you no longer control another attacking or blocking creature.
9/29/2017 If an effect says that Bonded Horncrest attacks or blocks if able and you control another creature able to attack or block, you must attack or block with Bonded Horncrest and that creature.
9/29/2017 In a Two-Headed Giant game, Bonded Horncrest can attack or block with a creature controlled by your teammate, even if no other creatures you control are attacking or blocking.