The Indus Dictionary Project
This is a pictograph of a hoofed animal's leg combined with two diagonal lines that resemble the Chinese character bā 八. Bā can mean all around or on all sides, which is the interpretation that I think applies here. Hence, Mahadevan's sign number 85 is an ideograph for the sound of reverberating hooves.
Harappa: Seal: H-447 a (NB. It is easier to see this symbol in image A): Sayid Ghulam Mustafa Shah and Asko Parpola, 1991: Corpus of Indus Seals and Inscriptions: Volume 2: Page 276: Collections in Pakistan: Suomalainen Tiedeakatemia.
There is only one extant example of this Indus sign.
Indus Script Sign Number 85: Sign List of the Indus Script: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.