'equal-petaled' is a modern English compound formed from 'equal' + 'petal'. 'equal' comes ultimately from Latin 'aequalis' meaning 'level, even', and 'petal' comes from Greek 'petalon' meaning 'leaf' (via New Latin/modern botanical usage).

'equal' entered English from Old French and Middle English forms derived from Latin 'aequalis'; 'petal' was borrowed into English in the 17th century from New Latin/Greek 'petalon' (meaning 'leaf' or 'thin plate') and was applied to flower parts, forming compounds like 'equal-petaled' in botanical descriptions.

Individually, 'equal' originally meant 'even' or 'level', and 'petal' originally meant 'leaf' or 'thin plate'; together in modern botanical usage they mean 'having petals of equal size/shape'.