adding an element type for an outlier ability?

how do you handle enemy abilities of a type that are rare in your game, but they don't really fit in with any of your normal element types? As an example, a few monsters and maybe a boss could have a skill that spits acid at the player. Would you add in a new element type called "Acid" ? Or just give it no elemental property and make it a neutral-based magic attack?

depends on what the attack should do and how it is supposed to be defended against.

The function of elements is to give different battlers different damage rates for this type of attack, meaning that some might be resistant or weak against it and receive more or less than the stated damage.
If that is the case (or if you consider making that the case with future battlers), then go the element route.
If that will never be the case, then don't - because it would only clutter up your element list.

Could make a Special element that covers all of the outliers.

@jonthefox I am unsure if this is a general concept question you are asking or if you are asking how to do it/what's the best way to do it.

Could you please post and clarify.
Thanks.

concept question

Okay, thanks.

From a mechanical standpoint elements generally have two purposes. To apply weakness and resistance effects (usually damage), and to act as a 'designator' for special effects.

An example of a 'designator' would be if a tree monster gets hit by a fire spell it 'transforms' into a new enemy (fire tree). It would matter less if the tree had a weakness to fire but whether or not the skill itself was classified as being from that element.

So the question to ask yourself is: are there currently (or going to be) any specific interactions to 'acid' in your game? Any skills which negate/prevent/reduce acid? Any cases where a special effect happens if hit by acid?

Do note that adding states outside of special 'reactions' is normally done on a skill by skill bases, so even if you declared "acid skills can inflict the
'chemical burn' state" this doesn't mean you would need a 'acid' element unless there were to be an actual reaction to the element itself.

i.e. lets say there were a particular piece of armour made out of baking soda (don't ask), which would 'dissolve' if hit by an acid skill regardless of the 'chemical burn' state.

It depends to some extent on what the purpose of the Elements system is in your game.

For example, in one of my games the elements are mostly for flavor. There are a few skills/equips that interact with the elements - for example an armor that gives you a buff each time you get hit with a different element within a battle. For stuff like this, I'm fine with creating a new element for Acid (even if only one or two enemy skills in the game use it) and allowing that equip to give you the buff for taking Acid damage.

For games where the elements system is more essential to gameplay, I don't think there's a lot of wisdom in creating new elements for these edge cases. It can be confusing for the player, and forces you to design around the new element (e.g. if there are five elements, that's 20 different attack-defense interactions, but if there are three edge-case elements added in, now there are 56 different combinations you have to design around). And if you're designing UI to present information to the player up-front (which you should whenever your elements system is important to combat), now you have to complicate the UI with a lot of elements the player will rarely interact with.

In games like that (where elements are essential to gameplay), I would most likely try to shoehorn the outlier ability into one of the existing elements - for example, an swamp slime or frog spewing out acid for Acid damage could be shoehorned into Nature (the idea of biochemicals eating away at something), or even shoehorned into Water (similarly to many poison attacks - the idea that the Water element covers all liquids). If it felt really bad to try to shoehorn it, then I'd either make it neutral non-elemental damage, or I'd simply rework the attack's name and aesthetics into something else.

If you want some of your party members to be either weaker, immune, stronger or absorbance to the acid attack then yes do make an element for it. If one of your party members also has a skill like this and you want the same thing but on different enemies instead of your party members, then again, do make an element for it.

Otherwise, I don't see a visual problem here. I don't think this element would have to have any visual icons or extra visual effects showing it's a different element/no element, so if that's the case in your game, your fine leaving it as no element. Then all you have to do is choose whether you want it to be physical/magical/certain hit/etc.