I love the nickname [name_m]Leo[/name_m], but don’t think it could be a full first name on it’s own. What names could [name_m]Leo[/name_m] be a nickname for? Please no [name_m]Leonardo[/name_m]/[name_m]Leonard[/name_m]
Whilst Leo can be used as a nickname or a short form, it actually originated as a full name. Leonard and Leonardo arose independently - although they share the lion meaning, that element is Leon, not Leo. They are more Germanic names, where Leo is pure Latin (although of course Leon and Leo are closely linked in terms of etymology). There have been loads of popes called Leo, and they never ever have nickname names - you’re not going to find Pope Dan or Pope Jerry. If you like Leo, there is no reason to try and lengthen it just because.
[name_m]Leo[/name_m] - just [name_m]Leo[/name_m] - was the name of popes and saints as well as as literary giants like [name_m]Leo[/name_m] Tolstoy. [name_m]Leo[/name_m] can stand on its own quite nicely.