'tadpole-shaped' is a Modern English compound formed from the noun 'tadpole' + the adjectival suffix '-shaped' (meaning 'having the form of').

'tadpole' comes from Middle English 'taddepol' (also attested as 'tadepol'), itself from Old English elements meaning 'toad' + 'head' (originally something like 'toad-head'); 'shape' is from Old English 'sceap'/'gesceap' meaning 'form' or 'created form', and the past-participle/compound pattern '-shaped' developed in later English to form adjectives. These parts combined in Modern English to produce 'tadpole-shaped.'

Initially, the elements referred to 'toad-head' (the head-like appearance of larval amphibians), and over time 'tadpole' came to mean the aquatic larva of frogs and toads; 'tadpole-shaped' therefore came to mean 'having the form of a tadpole' in modern usage.