One of the player characters in a game I run is a halfling rogue. I want to give her a non-evil shadow mastiff as an intelligent mount due to synergy with her rogue features. This is a case where the Rule of Cool will bring my player some happiness and a unique edge, so I am not interested in being told I am wrong for trying this (even if I am). Rather, this question will be about a matter of balance.
The shadow mastiff has a feature called Shadow Blend, described as follows (Volo's Guide to Monsters p. 190):
Shadow Blend. While in dim light or darkness, the shadow mastiff can use a bonus action to become invisible, along with anything it is wearing or carrying. The invisibility lasts until the shadow mastiff uses a bonus action to end it or until the shadow mastiff attacks, is in bright light, or is incapacitated.
Since the rider does not count as a thing being worn or carried, the rider does not turn invisible with the shadow mastiff when it uses this feature (as affirmed by a related question). Therefore, using Shadow Blend as written leads to the case where a rider mounted on a shadow mastiff stays visible while its mount becomes invisible, and that strikes me as silly and immersion-breaking. Let's call that the "invisible jet" effect after the appearance of Wonder Woman's invisible jet.
How can I effectively avoid the "invisible jet" effect when a shadow mastiff is being used as a mount?
I am looking for a GS/BS answer describing a house-rule that modifies the text or interpretation of the Shadow Blend feature just enough to avoid the "invisible jet" effect.
However, it needs to have been tested at a table in similar circumstances (relating to any sort of invisible mounts, not necessarily to shadow mastiffs in particular) and confirmed to be balanced and effective by reasonable metrics.
Best Answer
Describe it differently
The crux of this seems to be "Wonder Woman's Invisible Jet is silly," which is an issue with the fiction the rules inspire. I agree, it's a pretty silly thing to have to imagine. In this case, though, let's ask ourselves the question: do I have to imagine it that way?
We're talking about a beast that blends with darkness and shadow, not a wizard's Invisibility spell or a Predator cloaking device. If the whole problem is the idea of a halfling awkwardly floating above the ground, legs akimbo — well, imagine something else.
Describe it as "I'm riding a smokey blob of shadow," or "I'm riding a weird beast-silhouette with edges that waver like dark fire," or "I'm riding a regular ol' dog but its coat blends so well that you can't figure out where the dogs ends and the shadows begin til it's in full motion jumping out at you like RAWR." I promise nothing important will break if you say that you can't literally see through the big magic darkness-blending shadow-dog to the rider's leg on the other side.
Mechanically, you don't need to change much: the dog is still "invisible" as far as the rules are concerned, and the rider isn't. Feel free to describe being able to sorta make out the dog's shape or presence while it's got a rider, since just seeing the rider is enough for enemies to know its location and take a swing at it even if it's invisible. Feel free to describe how the rider is partially obscured by the dog's fur or its aura or whatever, but you can see most of them pretty clearly (that doesn't require special mechanics any more than "this broad-brimmed hat I'm wearing casts a shadow over my face" requires special mechanics).
There may be some small points where the mechanical and fictional description diverge, but in my experience that'll happen with detailed rules for seeing things in D&D across editions anyway. Just invoking the detailed concealment or invisibility rules has a tendency to bring out goofy edge cases.
(Experience: that's how we've treated shadow-creature shadow-invisibility whenever it's come up across different editions of D&D and it works pretty fine. Not transparent, just impossible to isolate from their environment.)