So I saw this on My Little Brony:

And it reminded me of this:

...
I am so creeped out right now.

I'm finished...
I finished Background Pony...
Just... give me a minute...
*sniff*
I need to take a walk...

In my opinion, different facets of the natural universe, or domains, choose a pony/zebra/griffon/other sentient creature to represent that domain. This representation is represented by a change in their physiology. Usually the chosen creature is a pony, since they are most in tune/attuned to nature and emotion.

For example:
Celestia: the Sun representing Light and Warmth
Luna: the Moon representing Rest and Comfort
Cadence: the Crystal Heart representing Love and Romance

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