Beau - belle
These are two forms of the same French word meaning "beautiful". In its masculine form, it is beau, pronounced like 'know' (IPA: /bəʊ/). In the feminine, it is belle, like English 'bell' (IPA: /bɛl/}. The "belle of the ball" (and anywhere else - the loveliest present) is always a woman; a "beau" (nowadays mostly found in historical novels, and older fiction) is always a man. The beau is usually the young man who is courting, near to courting, or supposed to be about to start courting the young woman at the centre of the story.
The plural of beau is properly the same as that in its original language: beaux. However, beaus is to be seen. No doubt if the word beau comes to be used more commonly (which seems unlikely), it will be fully assimilated into English, and beaus will come to seem normal, beaux merely quaint.
The prefix beau has been absorbed into several English proper nouns, the names of places or of families, and the common English adjective beautiful. Like the adjective, the proper nouns are not always easy to pronounce. Beauchamp, for example (French for 'beautiful field') is pronounced 'Beecham' (IPA: /ˈbiːtʃ əm/) - as it is written by some branches of the family. The prefix is sounded in roughly the French way (/əʊ (or o)/) in the family names 'Beaufort', 'Beaumont' and 'Beauclerk' (/ˈbəʊ klɛə/). In the place-name 'Beaulieu' (the French for 'beautiful place'), it is spoken like 'beautiful', /ˈbjuː lɪ/.
See also odd pronunciations of proper names.