Your comment makes no sense. If it was designed for non-null terminated strings, why would it specifically pad after a null terminator?

I looked up the actual reason for its inception:

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    Rationale for the ANSI C Programming Language", Silicon Press 1990.

    4.11.2.4 The strncpy function
    strncpy was initially introduced into the C library to deal with fixed-length name fields in structures such as directory entries. Such fields are not used in the same way as strings: the trailing null is unnecessary for a maximum-length field, and setting trailing bytes for shorter names to null assures efficient field-wise comparisons. strncpy is not by origin a "bounded strcpy," and the Committee has preferred to recognize existing practice rather than alter the function to better suit it to such use.