Wikipedia says that "Bulgar (also Bulghar, Bolgar, Bolghar) is an extinct Oghur Turkic language which was spoken by the Bulgars. "
I was not aware of a different hypothesis on this until I answered this question and, reading this answer I have found this linked article called Y-Chromosome Diversity in Modern Bulgarians: New Clues about Their Ancestry which concludes:
On the whole, in light of the most recent historical studies, which indicate a substantial proto-Bulgarian input to the contemporary Bulgarian people, our data suggest that a common paternal ancestry between the proto-Bulgarians and the Altaic and Central Asian Turkic-speaking populations either did not exist or was negligible.
This line of argument is not impressive. The factual result of the genetic inquiry is that the present Bulgarian gene pool has no significant Turkic ancestry. But a conclusion that the same is true for the proto-Bulgarians can be reached only if "a substantial proto-Bulgarian input to the contemporary Bulgarian people" was a proven fact. (Which is not. The hard fact of present gene pool can be interpreted as insignificant proto-Bulgar ancestry in present Bulgarians.) Also, the conclusion mixes in one phrase genetic and linguistic ancestry.
I think that the question of "a substantial proto-Bulgarian input to the contemporary Bulgarian people" is very difficult to solve, and I imagine it cannot have a clear and definitive answer. What interests me here is a different one — because the same article also states:
Novel analyses of proto-Bulgarians epigraphic monuments, especially, of the major historical inscription – “the List of the Bulgarian Khans” - have revealed that the proto-Bulgarian language did not belong to the Turkic linguistic family... ...for grammatical features the proto-Bulgarian language gravitates towards the Pamir languages of the East Iranian group, which belong to the Indo-European branch
I think that this question about the categorization of the (proto) Bulgar language can have a clearly cut answer.
The linked bibliography for that is:
Pritsak O (1955) Die Bulgarische Fürstenliste und die Sprache der Protobulgaren. Wiesbaden: Ural- Altaische Bibliothek, I.
Mänchen-Helfen O (1973) The world of the Huns. Studies in their history and culture. Berkeley: University of California Press.
Dobrev PD (1991) The Proto-Bulgarians - origin, language, culture. Sofia: Proxima.
Dobrev PD (2005) The golden core of the Bulgarian antiquity. Sofia: Tangra TanNakRa IK.
Dobrev P (1994) The world of the proto-Bulgarians: realities and misunderstandings. Sofia: IKK Slavika – RM.
Page numbers are not mentioned.
To what extent this hypothesis (that the language of Bulgars was not Turkic, but Indo-European) is accepted in mainstream scholarship?