Cognomen Horncastle (Horncastle 
) , worldwide, is a rare surname. The family name Horncastle is habitual in New Zealand, where it is a rare surname, the United Kingdom, and Canada. Measured by absolute frequency, it is commonest in the United Kingdom, Canada, and the United States.
| VARIANT | COUNTRY |
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| Horncastle | United Kingdom, Australia, United States, Canada, New Zealand |
| william horncastle British cricketer (1864-1917), GB (b. 1864) |
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| john horncastle Bishop of Carlisle elect |
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| nicholas horncastle (d.1419) of Northampton |
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| emma horncastle researcher |
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| edward thomas horncastle Ph.D. University of Liverpool 2008 |
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| mona horncastle (b. 1973) |
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| tristan horncastle Canadian singer, CA (b. 1986) |
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| james horncastle (b. 1987) |
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| james horncastle English sportswriter and broadcaster |
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