Oddball, Experimental, or One-off

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The Supermarine Type 508 was a British experimental jet fighter prototype developed by Vickers-Supermarine in the early 1950s, serving as the primary ancestor to the Supermarine Scimitar.  It was designed to meet the Admiralty's Specification N.9/47 for a carrier-borne interceptor, featuring a straight-wing configuration, twin Rolls-Royce Avon RA.3 engines, and a distinctive "butterfly" V-tail designed to clear jet exhaust while maintaining stability. 

Key Development and Specifications

Maiden Flight: The first prototype, serial VX133, flew on 31 August 1951 at Boscombe Down, followed by a second prototype (VX136, later designated Type 529) on 29 August 1952. 
Design Origins: The Type 508 evolved from the Type 505, which was originally intended to land on flexible rubber decks without an undercarriage; the Type 508 incorporated a conventional tricycle undercarriage after the Admiralty abandoned the rubber deck concept. 
Performance: The aircraft achieved a maximum speed of approximately 607 mph (977 km/h), though this was considered modest, leading to the development of the swept-wing Type 525 variant. 
Operational History: The Type 508 (VX133) conducted carrier trials aboard HMS Eagle in 1952 and remained in service until 1956, while the Type 529 variant was destroyed in a crash landing on 19 December 1953.