Funny coincidence — I was just digging through an old manga wiki last night and this exact kind of question came up in a thread.
The tricky part is that 'the scorpion' could mean different things depending on the series: a character literally named Scorpion, a creature that looks like a scorpion, or a symbolic scorpion motif. If you tell me the manga title I can be specific, but if you want to hunt it down yourself, start by checking chapter summaries on fandom wikis and sites like MangaUpdates or MyAnimeList. Search the page for the word 'scorpion' or scan chapter titles; many wikis list first appearances and episode/chapter numbers.
Also look at the publication history: check volume tables of contents, official English releases from the publisher, and author notes in tankobon extras. If the manga ran in a magazine, back issues or magazine indexes can pinpoint the exact chapter and date. Tell me the title and I’ll chase down the exact chapter and release date for you — I love this kind of scavenger hunt.