Data in the Creation Kit indicates that this spell was originally intended to be a flaming draugr. In fact, unlike the actual familiar, the flaming familiar has neither the keyword to make it a familiar nor the keyword to make it a daedra.
This also means it spawns with an Ancient Nord Sword or an Ancient Nord War Axe; it can equip this, as it has a right hand, and it may choose to do so, as it has 15 ranks in One-handed and uses the Draugr combat style, not the Familiar combat style.
Because it is not considered either a daedra or a familiar, it cannot be turned against you by enemy Conjurers or Dragon Priests.
https://en.uesp.net/wiki/Skyrim:Flaming_Familiar
As you can see, the Flaming Thrall has a bunch of leftover and wrong data. 0009ce28 SummonFlamingThrall should have "LItemDraugr02Weapon1H [LVLI:000242FC]" removed. It should have the "ActorTypeDaedra [KYWD:00013797]" and "ActorTypeFamiliar [KYWD:0010EAD7]" keywords added. Class should probably be changed to "EncClassAnimalPredator "Predator" [CLAS:000131E6]" and combat style to "csFamiliar [CSTY:000B729C]"".
Maybe the name should also be changed to Flaming Familiar, to fit the name of the spell.