Description

For this quest you have to throw an item at a Rejuvenated Thunder Lizard. After then killing the lizard, a blood filled leech will spawn near the body that you can pick up. Neither the lizard nor the leech shows up as mobs of interest for the quest, or show up on the map where to find them.

Wowhead link

Quest: https://www.wowhead.com/cata/quest=14304/blood-theory
Lizard: https://www.wowhead.com/cata/npc=35412/rejuvenated-thunder-lizard
Leech: https://www.wowhead.com/cata/item=49136/blood-filled-leech#created-by-spell

Questie & WoW version

Questie v10.3.5 & Cataclysm Classic