Question from a DM: If a Tiefling and a Hobgoblin mate, what offspring does it produce? The situation is that a PC and NPC relationship looks like it will produce offspring. If there isn't explicit rules text, a well supported answer citing dungeons and dragons lore is acceptable.
There are a couple novels which reference tiefling children, one being the Neverwinter sub-series of the Drizzt series (Elf + Tiefling = Tiefling) and the Brimstone angel series:
Prior to Asmodeus's ascension to godhood, the infernal blood could be diluted through intermarriage, but afterward, the union of a tiefling with another race always produced a tiefling child. - Forgotten Realms wiki, referencing the Brimstone Angels: Lesser Evils novel.
I am not sure how authoritative novels can really be considered, but the DM always can decide however they would like in such manners.
It might also be worth noting that hybrids are quite rare and seem to be limited to only a few races, namely Humans.
Basically, the DM would have to interpolate. I would say that the offspring would likely be (or at least have a decent chance of being) tiefling hobgoblins, but might just be regular hobgoblins instead. You'd have to decide the chances.
Hobgoblin PCs, according to the Volo's Guide, get +2 Con, +1 Int, darkvision, proficiency in 2 martial weapons and light armor, the "Saving Face" trait, and speak common and goblin.
Tiefling PCs, from the PHB, get +2 Chr, +1 Int, darkvision, fire resistance, the "Infernal Legacy" trait, and speak common and infernal.
So you can mix and match from there.
I would describe it as looking like a hobgoblin with horns and a tail, and go with: +1 Con, +1 Int, +1 Chr, darkvision, and fire resistance. I would then choose (or have the player choose), for each individual, whether to have (a) the proficiencies, "Saving Face", and speak goblin, or (b) "Infernal Legacy", and speak infernal, depending on which parent was more influential in its upbringing.