From ErfWiki

Proposed Canon

A type of Dwagon.

They, along with pink and purple, are described as one of the weaker varieties of dwagon.^{Erf-b1-p062, Erf-b1-p064}

Speculation

Its special attack, unlike that of other dwagon types, is not breath attack. Apparently yellow dwagons defecate (acidic battlecrap) on their victims.^{Erf-b1-p006, Erf-b1-p031}