This is the outline of an unborn baby, with the addition of four legs. Hence, it is an ideograph for a young animal.
Illustrative Text Reference:
Harappa: Seal: H-24 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 173: Collections in India: Suomalainen Tiedeakatemia.
Image Credit:
Indus Script Sign Number 1058: List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.