In my setting tieflings are mostly a product or in some cases a side-effect of fiendish/infernal magic. It might be that the unborn baby gets cursed in the womb or was one of the parents cursed to sire a tielfling. In other cases proximity to infernal magics or having been at some time affected by them can cause a parent to have tielfling childern. Fiendish parents are rarer, but happen on occasion.
In addition i use both the PHB option and the feral option from SCAG (+2 dex instead of cha). The "Charismatic ones" have only subtle or easily hideable signs of their ancestry, while the "ferals" tend to have the "horns and tail" look. Although there are occasional exceptions to that. he charismatic/subtler ones tend to be more common, mostly because many of the more fiendish looking are often abandoned or killed afterbirth. In general tielflings are disliked and aren't protected by any laws (basically you could go and stab one in broad daylight and would get at away with it). Most prefer to hide their true nature, some more obvious looking going so far to attempt to remove their tails and horns or alter them in a way to try to pass as one of the feytouched (half elves of my setting), who in some cases might have some horns.
In addition i use both the PHB option and the feral option from SCAG (+2 dex instead of cha). The "Charismatic ones" have only subtle or easily hideable signs of their ancestry, while the "ferals" tend to have the "horns and tail" look. Although there are occasional exceptions to that. he charismatic/subtler ones tend to be more common, mostly because many of the more fiendish looking are often abandoned or killed afterbirth. In general tielflings are disliked and aren't protected by any laws (basically you could go and stab one in broad daylight and would get at away with it). Most prefer to hide their true nature, some more obvious looking going so far to attempt to remove their tails and horns or alter them in a way to try to pass as one of the feytouched (half elves of my setting), who in some cases might have some horns.
