The term sea urchin suggests the existence of a non-sea urchin, but the only use of just plain urchin that most of us have ever heard is in reference to, say, the Artful Dodger — but that was a cute repurposing of an older term:
1 old-fashioned: a mischievous and often poor and raggedly clothed youngster, street urchins
2: SEA URCHIN
3 archaic: HEDGEHOG sense 1a[…]
The word urchin in its original sense has been largely replaced by HEDGEHOG in standard British and North American English. Despite this recession, the Survey of English Dialects showed that urchin in various phonetic manifestations, with variants such as prickly-urchin, was still in dialect use in the west Midlands and north of England in the 1950′s (see Survey of English Dialects: The Dictionary and Grammar, Routledge, 1994). The application of urchin in a more or less pejorative way to a child, much more rarely to a young woman, began in the sixteenth century; according to the Oxford English Dictionary, first edition, it became more common after ca. 1780.