Can a Tiefling Subrace be Feral?

Question

Mordenkainen's Tome of Foes offers the option to select a different heritage for a Tiefling by replacing the default Ability Score Increase with its own (as well as by replacing the Infernal Legacy trait):

The traits of the chosen subrace replace the tiefling’s Ability Score Increase and Infernal Legacy traits given in the Player’s Handbook. 

Sword Coast Adventurer's Guide offers variant tieflings by either replacing Infernal Legacy (for Winged, Hellfire, or Devil's Tongue) or by replacing one Ability Score Increase trait with another increasing feature (Feral):

Feral... This trait replaces the Ability Score Increase trait.

While a non-Asmodeus Tiefling subrace is not compatible with Winged, Hellfire, or Devil's Tongue (since they both replace Infernal Legacy with their own option), a non-Asmodeus Tiefling subrace may be eligible for Feral (by replacing the subrace's own Ability Score Increase with the Feral one). Is it?

Answer 1

It's possible with DM permission.

The Feral trait is described as being allowed by DM permission in the sidebar on page 118 of the Sword Coast Adventurers Guide, as follows (emphasis mine):

Since not all tieflings are of the blood of Asmodeus, some have traits that differ from those in the Player's Handbook. The Dungeon Master may permit the following variants for your tiefling character...

Assuming the DM is open to it, there's no reason that a Levistus Tiefling, for example, doesn't quality as a "tiefling character" for purposes of variant traits including the Feral trait for ability scores. There's nothing in either SCAG or Mordenkainen's Tome of Foes that addresses feral tieflings at all beyond that one sidebar, so there's no lore indicating that a non-Asmodeus tiefling can't be feral. All we're told is that the PHB tiefling is an Asmodeus Tiefling (this is reinforced in MTOF on page 21) and that not all tieflings are of the blood of Asmodeus. It doesn't actually say that the variant options are restricted to non-Asmodeus tieflings, and even if that seems to be implied, note that that passage was written before MTOF ever presented any other non-Asmodeus options.

So, with DM permission, the combination should be possible (even though, as I explained in a related answer to another question, it's not technically possible using D&D Beyond's tools).