Full Idea
In considering questions of real possibility, it is important to keep the distinction between what a thing is and what it looks like clearly in mind. There is a possible world containing a horse that could then look like a cow, but it wouldn't BE a horse.
Gist of Idea
One thing can look like something else, without being the something else
Source
Brian Ellis (The Philosophy of Nature: new essentialism [2002], Ch.6)
Book Reference
Ellis,Brian: 'The Philosophy of Nature: new essentialism' [Acumen 2002], p.113
A Reaction
This is an interesting test assertion of the notion that there are essences (although Ellis does not allow that animals actually have essences - how could you, given evolution?). His point is a good one.