This is a pictograph of a pack. Inside the pack is the outline of a mane from a stylized horse. It is an ideograph for a pack that is transported by equines. Hence, it is the noun donkey load or
Illustrative Text Reference:
Harappa: Seal: H-661 a: Sayid Ghulam Mustafa Shah and Asko Parpola, 1991: Corpus of Indus Seals and Inscriptions: Volume 2: Page 309: Collections in Pakistan: Suomalainen Tiedeakatemia.
There is currently no archaeological evidence for the existence of mules in the Indus Civilisation.
Indus Script Sign Number 1430: List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.