The Acid Dragon is the second boss in Aliens Of Gold. It is a large slug-like mutant that shoots acid. The Acid Dragon is fought in a large open area, and its acid can be easily dodged by strafing.
In the sequel Planet Strike the Acid Dragon's role was changed to a regular enemy and there were many more of them spread across the levels with much less health. On some of the later levels there is also stealth variants which are harder to see.