Ultimately, this comes down to you and your DM. The tails on Tieflings exist purely as flavour - they have no mechanical capabilities - so having a shark tail or axolotl tail bears no mechanical changes, just don't expect to swim faster! Dragons tail, the same - no reason your tiefling character can't have dragonesque features, but don't expect to say "I have spikes on my tail so now I can attack with it".
99% of the time, reflavouring is ok. The only times it gets stopped is if it contradicts the fluff or feel of the world, EG saying "My monk is actually a dude plugged into the matrix who's quest is to find a phone so he can get out" in a high-fantasy world is going to see people feel like you're not embracing the narrative they want to build.
It's all flavor. The mechanical features are clearly outlined in the PHB, and alternative "sub races" are listed in SCAG, but the visuals of a tiefling are down to you and your DM. In general, Tieflings have features reminiscent of fiends, which CAN include, but isn't limited to:
Horns
Unique skin tones (red, blue, etc.)
Tails, often pointed or forked
Pointed ears
Forked tongues
Unique eye color and pupil shape, or even solid color without a pupil
Goat hooves
An odor of sulfur
Arguably more so than other races, tieflings can be heavily customized. They can have any of these listed traits, or pretty much any other fiendish traits you can come up with. It's just a matter of talking to your DM.
I'm specifically wondering about tails. Can a tiefling have a shark-like tail? An axolotl tail? Maybe even a dragon tail?
Ultimately, this comes down to you and your DM. The tails on Tieflings exist purely as flavour - they have no mechanical capabilities - so having a shark tail or axolotl tail bears no mechanical changes, just don't expect to swim faster! Dragons tail, the same - no reason your tiefling character can't have dragonesque features, but don't expect to say "I have spikes on my tail so now I can attack with it".
99% of the time, reflavouring is ok. The only times it gets stopped is if it contradicts the fluff or feel of the world, EG saying "My monk is actually a dude plugged into the matrix who's quest is to find a phone so he can get out" in a high-fantasy world is going to see people feel like you're not embracing the narrative they want to build.
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Ultimately most races can be reflavored in a number of ways as long as you don't expect these to change them mechanically.
(For example, a lot of people reflavor Kenku as any number of different birds despite the basic version looking like a raven.)
Ask your DM.
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It's all flavor. The mechanical features are clearly outlined in the PHB, and alternative "sub races" are listed in SCAG, but the visuals of a tiefling are down to you and your DM. In general, Tieflings have features reminiscent of fiends, which CAN include, but isn't limited to:
Arguably more so than other races, tieflings can be heavily customized. They can have any of these listed traits, or pretty much any other fiendish traits you can come up with. It's just a matter of talking to your DM.
On the flip side your tiefling can have none of these traits. As far as the book says a tiefling can look totally human