Pseudodragons are Tiny CR 1/4 dragons, but they're not quite dragon-like enough for my purposes. I want to homebrew some alternative stat blocks to the pseudodragon so that they more so resemble the various metallic/chromatic dragons, but at the same time I'd prefer it if my changes didn't change their overall CR. Specifically, I want to add damage resistance and a breath weapon, similar to "true dragons", resulting in 10 variations of the pseudodragon, and I want these 10 variations to still all be CR 1/4.
To that end, I have swapped out their Magic Resistance trait for the damage resistance that relates to their colour (as per the Draconic Ancestry table of the dragonborn playable race, for example) and have swapped out their Sting attack for a Breath Weapon attack (again, taken directly from the dragonborn playable race, assuming the lowest possible number of damage dice; i.e. 2d6 from a level 1 dragonborn PC). I'd have the DC for the breath weapon be the same as the DC for the Sting attack, and I'd have it on a Recharge 6, same as, say, a dust mephit's Blinding Breath.
If I were to make these changes to the pseudodragon, would it still be the same CR? Is this true for each "colour" pseudodragon (i.e. does fire damage vs. cold damage make one variant inherently more powerful than the other, etc)? I'm hoping that they're all the same CR (ideally CR 1/4), regardless of the damage type resisted/dealt via breath weapon…
If this isn't too much for one question, assuming that they are all balanced against each other regardless of damage type, as I hope, then if I were to add a 15 ft. swim speed and the Amphibious trait of certain dragons that can swim and breathe underwater (such as gold dragons) to the corresponding pseudodragons (i.e. my gold pseudodragon), would that make much of a difference to their CR vs. those that would not have those traits (i.e. my red pseudodragon)?
I'd probably also want to add a 15 ft. climb speed to the pseudodragons that are of the same colour as true dragons that have climb speeds, i.e. my red pseudodragon (although given that pseudodragons can fly, I'm guessing this is of no consequence to the CR, but I thought I'd mention it just in case it matters).
Best Answer
Their calculated CR remains the same, but they are far stronger
Carcer rightly points out the way your CR is calculated, but I feel like that doesn't capture the entire picture.
I feel like this pseudodragon may very well be punching above its 1/4 weight with a 2d6 breath weapon. With a lucky roll, there's a good chance it could knock out an entire first level party in a single breath weapon. CR 1/4 indicates it should be roughly 2-3 pseudodragons to form a decent challenge for a first level party, but 2-3 pseudodragons with this breath weapon will absolutely annihilate any first level party round one if they know how to aim.
Ogres suffer from a similar problem. If you calculate their CR, it comes down to a CR 1 creature, but they are still listed as a CR 2 creature, simply because there's a really good chance that it'll oneshot a first level character.
On top of that, you've replaced their attack which requires getting in melee range with a breath attack they can use from a distance. This, combined with a fly speed of 60, means they can very easily fly into range, breath weapon, fly away, and then fly away until their breath weapon has recovered, before coming again for a new run.
This is at a level where most characters will have very few ways of dealing with a flying opponent.
I for one would not want to face even one of these creatures at level 1.
If the breath weapon is changed to be once per short rest instead of Recharge 6, it'll at least make it a lot more manageable because it can't hit and run. Also reducing the damage to 1d6 may very well be enough to keep it around roughly the same challenge rating. It'll at least no longer be something I'd really dread fighting at level 1.
As an aside, regarding being used as a familiar (say, for a Warlock via Pact of the Chain), it's most likely not that big a deal, I doubt it'll be more impressive than invisible scouts. Familiars used for combat purposes tend to be dead familiars before long. It'll be good for killing some goblins every now and then, but it'll be costly to keep resummoning your dead familiar.