. ▁All ▁three ▁are ▁intellig ent ▁be ings ▁capable ▁of ▁interact ing ▁with ▁other ▁dem ig ods ▁independent ▁of ▁their ▁horse - to - human ▁transl ator , ▁Per cy . ▁ ▁Per cy ' s ▁second ▁mag ical ▁companion ▁is ▁Mrs . ▁O ' Le ary , ▁who ▁is ▁introduced ▁in ▁the ▁fourth ▁novel , ▁Battle ▁of ▁the ▁Lab yr inth . ▁She ▁is ▁a ▁hell h ound ▁that ▁he ▁receives ▁from ▁Da ed alus ▁( wh om ▁Per cy ▁meets ▁as ▁a ▁sword ▁master ▁named ▁Quint us ) ▁before ▁the ▁invent or ▁dies . ▁Though ▁Mrs . ▁O ' Le ary ▁is ▁described ▁as ▁" the ▁size ▁of ▁a ▁tank ", ▁the ▁mag ical ▁ve il ▁known ▁as ▁the ▁M ist ▁causes ▁her ▁to ▁appear ▁as ▁a ▁po od le ▁to ▁mort als . ▁Per cy ▁often ▁refers ▁to ▁her ▁as ▁" his ▁dog ". ▁Per cy ▁sometimes ▁uses ▁Mrs . ▁O ' Le ary ' s ▁ability ▁to ▁" shadow ▁travel " ▁to ▁cross ▁large ▁distances ▁almost ▁instant ane ously . ▁His ▁brother ▁T ys on ▁and ▁dead ▁friend ▁Charles ▁Beck endorf ▁are ▁Mrs . ▁O ' Le ary ' s ▁other ▁care t akers , ▁though ▁she ▁is ▁also ▁fond ▁of ▁N ico ▁di ▁Ang elo , ▁son ▁of ▁H ades . ▁ ▁He ▁also ▁knows ▁a ▁hi pp oc amp us ▁named ▁Rain bow ▁who ▁lik es ▁Per cy ' s ▁half - bro ther ▁T ys on . ▁It ▁first ▁appears ▁in ▁The ▁Sea ▁of ▁Mon sters