The answer is: Camel's hair

Matt 3:4 - A rough and coarse garment like the prophets used to wear. c.f. Zec 13:4; 2 Ki 1:8

John the Baptist's choice of attire—a garment made from camel's hair—brings to mind the rich tapestry of the Hebrew tradition. His rugged outfit not only set him apart in appearance but also symbolized his calling as a prophet. Much like Elijah, whose prophetic role was marked by a similar rough cloak (2 Kings 1:8), John serves as the voice of preparedness, channeling ancient prophecies into the fabric of his ministry.

Interestingly, camel hair has practical implications too; it was often used for making outer garments due to its durability and breathability. It also hints at a deeper meaning: John’s lifestyle was austere, living simply and calling for repentance in a world that valued luxury and comfort. This choice underscores deception in outward appearances, swirling in a society that often coddled temporal wealth while dismissing what is of eternal significance.

Moreover, in Zechariah 13:4, a prophetic passage references garments of hair, pointing to a time when false prophets would suffer shame for their misdeeds. Here, the stark contrast between linen fashion—symbolizing priestly roles—and John’s camel hide hints at his rejection of superficial ritual in favor of sincere transformation.

So, every time we meet this rugged prophet eating locusts and wild honey, we are reminded that the messenger sometimes appeared more eccentric than the message itself, calling humanity to bring heart and spirit to its faith with unmeasured zeal. John starkly represents that authenticity in spirit often clashes with the status quo.