Base Mosquito Girl and the crows as already mentioned, to begin with, although I wanna mention that the crows were not Demon level individually, but together. Beast King is a maybe and Dry DSK is a hard maybe, Genos would've beaten him one-on-one, but he was much stronger when he fought DSK. As well as:
Do-S-I doubt she can control someone as heavily modified as Genos to begin with, and obviously in terms of raw combat ability Genos is above her.
Macho Daikon-A bonus chapter Monster that was defeated by Smile Man and his 2 partners with relative ease.
Maybe Devil Long Hair-Defeated by Atomic's students with high-difficulty, it's vague how they compare to start of series Genos, but Genos might be stronger than all 3.
Probably Eyesight-The Medusa like Monster eaten by Pig God, she doesn't have a lot of feats at all, and her venom wouldn't work on Genos for obvious reasons.
Rafflesidon-The plant Monster with Senior Centipede. His gas probably wouldn't work on Genos, and I imagine he'd be weak to high heat from Genos due to being a literal plant. Not to mention it has pretty much 0 AP or durability feats.
Probably Rhino Wrestler-Barely any relevant feats and likely would've lost to Iaian over time.
Fist Fight Djinn-The poor boxing Monster who ran into Bang on a bad day. He has virtually no feats apart from knocking back but not severely harming Smile Man, who's just a mid A-Class Hero.
Free Hugger-Almost no feats and is centered around fights with physical melee opponents, whereas Genos uses almost only ranged attacks.
Probably Scaledon-A bonus chapter Monster. Best feat is defeating B-Class Heroes and was slightly damaged by a high tier B-Class Hero.
Maybe the Giant Crow-Was almost featless and it was stated by ONE that PPP could defeat it.
Probably Giant Snowman-The big snowman that showed up at the end of a bonus chapter. It's absurdly massive, but it's featless, and also it's literally a snowman, whereas Genos uses heat-based attacks, not the best matchup.