Reading a different question about the opposite of the word consultant made we wonder why the word shifted semantically from the asker to the person who answers?
consultant (n.) 1690s, "person who consults an oracle," from consult + -ant. In medicine, "physician called in by the attending physician to give consultation in a case," by 1872 (perhaps from French, where it was in use by 1867); general meaning "one qualified to give professional advice" is first attested 1893 in a Sherlock Holmes story. Related: Consultancy (1955).
The structure of the word suggests one who consults (others).
Consult means to ask or refer to. e.g. Consult the experts.
-ant (and its sister -ent) give the meaning of taking action or agency.
Attendants attend others. Commandants command others. Regents rule others. Dependents depend on others. Etc.
How did consultant shift from the person asking to the person asked?