From ErfWiki

Proposed Canon

First Appearance: TBFGK 56

A type of Dwagon.

They, along with pink and yellow, are described as one of the weaker varieties of dwagon.^Erf-b1-p058

Purple Dwagon's seem to be ideal for Siege as evidenced by their usage in the siege of Spacerock by Gobwin Knob where their Sonic attack easily blew off a set of large double doors and injured several Jetstone units behind it, including Prince Tramennis ^Erf-b2-p060

Speculation

The purple dwagon breath attack is sonic in nature.^Erf-b1-p079 The speculation is that the attack form is a "breath attack", since it is canon that the purple dwagons have a sonic attack ^{LIAB Text 36}

Real World References

Sonic Drive In is a fast food chain in the south-western United States with some franchises in the south-central USA and the south-east USA. It is typically has no indoor seating, with cars parking around a central kitchen, food is delivered to cars which park under a covered awning, commonly by servers wearing roller skates.