Lupus Optumus is a 
wolf summoned via a 
Beast Master's  Wolf Companion
 Wolf Companion ability, a variant of 
 Ranger's Companion
 Ranger's Companion. Lupus Optumus is the fully upgraded form of 
 Lupus
 Lupus and 
 Lupus Major
 Lupus Major once the Beast Master has reached level 11.
Because of the  Prey's Scent passive, Beast Master rangers may wish to attack the target marked by
 Prey's Scent passive, Beast Master rangers may wish to attack the target marked by  Hunter's Mark for optimal damage.
 Hunter's Mark for optimal damage.