It looks like a panther mixed with a fish, so as name I can think of:

• Aquapanther / Aquapanthera (Aqua + Panther)

• Baqueer / Baqueera (like Bagheera from the Jungle Book)

• Finther / Finthera (Fish + Panther)

I have a bunch of mutees and getting my first legendary one would be cool!