Thilo, Tilo and
Tielo (well, there are even more variants) are Northern
German short forms of names that begin with the element Diet-, like
Dietrich. According to my book on Frisian names, they are also East Frisian (in which case they are derived from Thiad-something names, but Thiad- is the Frisian form of
German Diet-).
Aw, this sounds really muddled, doesn't it? Sorry, it's late.