Kevo was an object-oriented language where classes were derived from objects rather than the other way around: a "family" of objects are ones that have the same signature. Families change dynamically as object properties are changed at runtime.

I haven't been able to find much detail about Kevo; it looks like it emerged around 1993 and its author is Antero Taivalsaari who has since done a lot of work since then on Mobile Java (J2ME). Here's an interesting paper that mentions Kevo:
Psychological criticism of the Prototype-Based OO-Languages