Monet is acually a harpy that ate the logia DF of snow (hey if there is an ice one, there can be a snow one). Why she looked human when Law first arrived is that as a logia, she made her wings and bird feet go into logia's elemental form (hers being snow) and then formed it to fit her needs. Like how Smoker can use his powers to make a single (or both arms) go all smoke form and still grab stuff …Monet is able to do that too but she made it look like arms and since its hard she was able to hold objects liek pens and books.
Why did she take the form back then and since kept the harpy form? She likes to make the henchmen guess if she's a human that ate the zoan harpy DF so they didn't know about her being a snow logia ...because she likes to play mind games
Actually this is semi serious. The part of her havign a snow logia is possible. REmember CC saying that Law didn't recognize her form as she spied on him?? Well a snow logia would easily blend in that frozen side that has slow falling.