Is it any coincidence Phillipa and filly have the same sounds? Maybe (since filly comes from Old Norse), but Phillipa does mean "lover of horses" in Greek, composed of philos (friendly love) and hippos (horse). It is the feminine version of Phillip/Philip, brought back to life in the 19th century. However, it seems Phillipa has never charted in the U.S. The strange thing is, everyone around the world was introduced to Pippa Middleton during the royal wedding coverage, and Philippa has been ever fashionable in England, but Phillipa was only used 10 times in 2011 in the U.S., the spelling Philippa used 53 times, Pippa used 69 times, and Felipa 8 times. That's pretty rare for a name everyone was raving about. But chances are these names will be used more in 2012, since the numbers did rise from 2010, when there were only 25 Phillipa's and 16 Pippa's born.
As for pronunciation, fil-LEE-pah is the most common, fil-IPP-ah the second most common, although it seems to make more sense phonetically that the spelling Philippa would make for a fil-IPP-ah pronunciation, and the spelling Phillipa would mean a fil-LEE-pah pronunciation. FILL-ip-ah is the third most common pronunciation. Pippa can be a nickname to either, as well as Flip or Filly/Philly. Pippa Middleton spells her full name Philippa. Both Pippa and Philippa have been used for literary characters: Robert Browning's poems "Pippa's Song" and "Pippa Passes," Libba Bray's character in "A Great and Terrible Beauty," "Pippi Longstocking," and a book by Rebecca Miller that Brad Pitt turned into a movie, "The Private Lives of Pippa Lee." Any spelling with an F - Filippa, Fillipa, Fillia, Fillipina, etc, and the variant Phillipine, Felepita, and Pelipa, are exceptionally rare.
In medieval times, Philip was fairly unisex, so the spelling Phillipa/Philippa, on paper, was used to determine which were female. Philip and Philippa also carried a note of wealth, as one had to be wealthy in order to own a horse or participate in horse related activities, which was true even in ancient Greece. One of the earliest namesakes was Philippa of Hainault (above), the queen consort of King Edward III of England, whose coronation was in 1330. There is some beautiful artwork of her and a story, "The Uncrowned Queen" by Anne O'Brien. She was well loved by her people, known for her kindness and compassion. She was also an excellent leader, serving as regent from time to time. Her name is a perfect example of Phillip being a unisex name at the time, because she was best known as Phillipe, not Phillipa. Fun fact: Chaucer's wife may have been named for Philippa of Hainault. A second medieval namesake was Philippa of Lancaster, queen consort of Portugal. She was born into royalty in England one decade after Philippa of Hainault died. Her marriage to King John I of Portugal secured the Anglo-Portuguese Alliance, and her children became so famous that they were known as the "Illustrious Generation." You can also find the name Philippa on a Duchess and a Countess.
Saint Philippa was martyred and crucified along with her son, Theodore, and others during the reign of Elagabalus, a Roman Emperor from 218 to 222. Blessed Philippa Mareri was strongly influenced by St. Francis and lived as Mother Superior in a convent in Italy. Blessed Philippa de Chantemilan and Blessed Philippa of Gheldre both lived in the late 1400s.
This name could appeal to those who like Lidia, Lydia, Portia, Phoebe, Fiona and the like, while Pippa could appeal to those who like Piper, Pepper, Poppy, or Fifer.