It has a glowing tail. It glows. That means it gives you light. Light is good.

Cute little lizard that likes to eat mushrooms

1 Glowtail Egg+ 3 Aquatic Mushrooms+1 Cooked Fish+Mejos+Fiber+Water= Glowtail Kibble

It can be tamed with the narcotic mushrooms like so players know

They have the best glow then of any of the glow tames. Babies eat meat not berries.