Earlier this week,
a post on Un-Comics* from The Monster Times ran the origin of the Rat. While the text promised the a follow-up tale, it never materialized within the pages of TMT. Eventually, however, Sutton & Flynn did return with the sequel in Arrgh! (as well as a full colour reprint of the original tale), as pointed out by commenter
D.D.Degg (Thanks, DDD!)
A comic sequel to an un-comic. Certainly a rare occurance, but was it unique?
I wonder...
Return Of The Rat by Tom Sutton & Peter A. Flynn for Arrgh! #3, (1975)
*(Un-Comics are what we call comics that come from sources other than comic books/strips)