* Yellowtail (fish): This is likely what you're thinking of. It's a popular name for several species of fish in the genus Seriola, particularly the Japanese Yellowtail (Seriola quinqueradiata). These fish are known for their bright yellow tail fins and are a prized food fish in many cultures.
* Yellowtail (butterfly): There's also a butterfly called the Yellowtail or Yellow-tailed Clearwing, with a scientific name of Euproctis chrysorrhoea. This butterfly is native to Europe and Asia and is known for its bright yellow tail hairs.
To figure out which "Yellowtail" you're looking for, please provide more context. For example, are you talking about a fish or a butterfly? Are you interested in its habitat, diet, or any other specific characteristic?