I believe the artist you are looking for is Richard Corben and the
story was called "Rowlf" (aka "The Story Of Rowlf"). It ran in three
issues of HEAVY METAL: November and December, 1979 and January, 1980.
This was its first appearance in colour.
The story originally ran in b&w in a hard to find fanzine called THE
VOICE OF COMICDOM in issue No.16 (1970) and No.17 (1971), and was
reprinted by Rip Off Press in 1971 and again in 1972, each with new
covers.
It was reprinted again in colour in January, 1987 in a trade paperback
called RICHARD CORBEN, COMPLETE WORKS Vol.3 (subtitled
"Rowlf/Underground 3").
If "Rowlf" isn't the story you had in mind, you might try looking up
Vaughn Bode, who often drew lizards and military equipment, or even
Kenneth Smith, who drew lizards ad nauseum. Both appeared in HEAVY
METAL frequently, though Bode "contributed" posthumously.