Pokemon name meaning:
don – Spanish Portuguese and Italian honorific title for a high ranking person.
A Japanese Pokemon name-meaning wiki, while affirming the Latin-language don meaning, states that “don” is also a frequently used suffix for monsters. Possibly this is a Japanese fictional convention, perhaps based on “mastodon”.
It has also been suggested that the “don” is taken from Mafia usage, though this seems unlikely to the present writer. The use of “king” names for final evolutions (in Japanese either オー [katakana for 王] or キング) seems clearly parallel to this use of “don”.