Dragon is a specific creature type.
From the Monster Manual, on creature types:
A monster’s type speaks to its fundamental nature. Certain spells, magic items, class features, and other effects in the game interact in special ways with creatures of a particular type. For example, an arrow of dragon slaying deals extra damage not only to dragons but also other creatures of the dragon type, such as dragon turtles and wyverns.
Dragons are large reptilian creatures of ancient origin and tremendous power. True dragons, including the good metallic dragons and the evil chromatic dragons, are highly intelligent and have innate magic. Also in this category are creatures distantly related to true dragons, but less powerful, less intelligent, and less magical, such as wyverns and pseudodragons.
It wouldn't be unreasonable to rule otherwise.
Above is the RAW ruling, but it seems perfectly reasonable for a DM to rule that the Dragon Scale Mail gives advantage against the chimera's dragon head breath weapon. It fits thematically, for sure, and the only difference in the chimera's and the red dragon's breath weapons is the word "head":
Fire Breath (Recharge 5–6). The dragon head exhales fire in a 15-foot cone. Each creature in that area must make a DC 15 Dexterity saving throw, taking 31 (7d8) fire damage on a failed save, or half as much damage on a successful one.
Adult Red Dragon
Fire Breath (Recharge 5–6). The dragon exhales fire in a 60-foot cone. Each creature in that area must make a DC 21 Dexterity saving throw, taking 63 (18d6) fire damage on a failed save, or half as much damage on a successful one.