To be honest, this week I wanted to do some deep-diving into acid and its role in nature. After about five minutes of reading, it turns out I haven't evolved much since the C+ I received in high school Chem.
So forget proton donors, let's shallow-dive into 5E's coolest acid damagers.
"Is this going to be a stand-up fight, sir, or another bug hunt?" Sorry, Hudson, looks like a bug hunt. In addition to its Dilophosaurus-like tendencies, the Ankheg is ready to cronch down on the nearest foe and melt its skin with acidic saliva. Some advice? Tip it over to get to the juicy parts (and lower its AC by 3).
Black Dragon
Does anyone else think this looks like something from the movie Spawn? Or is it Constantine?
Anyway, take extra care if you run across one of these big bois in your campaign. With a CR rating of FOURTEEN, you know you're in for a bad time. This idiot's Acid Breath attack will leave you feeling extra special, and extra tingly, on the outside.
Animated Breath (Acid Form)
As if that wasn't bad enough... Some dragons can practice hard enough to manage a physical, elemental manifestation of their breath!! I'm being serious, look it up!
Flumph
It is so, so, so hard to take these guys seriously.
But you should, seriously. Because it can cause you major bodily harm? Not really. Because it can outsmart you and trick you into doing its bidding? Also, no.
Because it's a sentient being that literally falls prone on the flip of a coin and is mostly harmless, and lawful good to boot? You got it.
With great power, comes great responsibility, to protect things that don't have great power... or something. I don't know, man. Just do the right thing and adopt this guy as your party mascot, alright?
Gelatinous Cube / Gray Ooze / Black Pudding / Ochre Jelly
We've seen spitting, we've seen breathing, and we've seen lashing out. Get ready for: digesting. Oh, lawd, they comin'. And they hungry.
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Though it is one of the rarer monster damage types you're likely to encounter, acid still poses a serious threat to life and limb (and light armor).
Be careful out there, kids. Lowered PH ain't nothin' to flumph with.