Learn English – Why is a bad bowler called a ‘pie-thrower’

Etymonline says:

ladybug

1690s, from lady + bug. The "lady" is the Virgin Mary (cf. Ger. cognate Marienkäfer). In Britain, now usually ladybird beetle (1704), through aversion to the word bug, which there has overtones of sodomy.

As to Lady Bird Johnson, that nickname was given to her by her nurse, as Wikipedia explains:

Though she was named for her mother's brother Claud, during her infancy, her nurse, Alice Tittle, commented, she was as "purty as a ladybird" [...]. That nickname virtually replaced her actual first name for the rest of her life. Her father and siblings called her Lady, though her husband called her Bird, which is the name she used on her marriage license. During her teenage years, her schoolmates had called her Bird, though mockingly, since she reportedly was not fond of the name.

Responding to your comment, I will add that both of her parents were natives of Alabama and the nurse was an African American. The Corpus of Historical American Language has these stats for ladybird vs. ladybug:

_{(X axis: year, Y axis: incidences per million words.)}

The earliest meaning for fountain shown by the OED is:

A spring or source of water issuing from the earth and collecting in a basin, natural or artificial; also, the head-spring or source of a stream or river. Now arch. or poet.

and a later meaning is "reservoir". When fountain pens began to be used, most pens were a quill only and had to be dipped into an inkwell every few words in order to replenish the ink. Hence, the new style of pen was called a fountain pen since it included a reservoir.