Beetle was never an official name, nor any of the other nicknames. It was first used by the New York Post in 1938 in an article describing the new car.

Before the war ended the car was officially known as the Kraft-durch-Freude-Wagen (KdF-Wagen), even though the VW symbol was also in use since 1938.
After the war the name Volkswagen was established as the brand name and first new models built from 1946 on (production in 1945 was from spare parts) were known as the Standard . In 1949 the better equipped version Export was added.

These names were used until 1961. From then on numbers depicting the engine size (1200; 1300 from 1965; 1500 from 1966) were used, while until 1965 the additional word Export was kept for the better equipped cars. In 1972 the larger 1302 and in 1974 the 1303 were added.

All the names known are just nicknames and vary from country to country. The English Wikipedia entry lists quite a few of them: http://en.wikipedia.org/wiki/Volkswagen_Beetle

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For us this means any extra nickname should be written in inverted kommas, e.g. 'Beetle' or 'Käfer'.