I was thinking (note the time)... are there horoscopes in equestria? 

We've had other examples of real life constellations in general like Star Hunter's Orion cutie mark. Both Ursas. And the latest one, although not actually equivalent to any real constellation, Rockhoof refers to it as O'Bridles Belt, which could be just a play on words with Orion's Belt, and we have too his story about how he tossed an Ursa mayor so hard into the sky it became a constellation ever since.

But what about Astrology in specific? Well, so far we only have one direct example of it and it's Twi's dress in "Suited for Success", where we can see clearly in her dress that she has printed the constellations of Scorpio, Aries, Cancer and Sagittarius.

(thanks shutterstock)

That's so cool! we found evidence! but how in equestria does that work inworld? 

We know that horoscopes in our world are to serve as a guide for life and prediction of someone's personality, behaviour, traits, strengths and weaknesses. They serve as a guide to know what to look out for in ourselves, in the things we do, and how to look for compatible others to have a better quality of life. So one can look to the stars for answers if they feel lost in their lives.
Knowing this then, how does this apply to equestrian lives? 

I would guess that like in real life, not everypony would believe in the power of stars helping you with how you live your life since, y'know, there's "actual" magic like the one from unicorns (and non-believers like Twilight in Pinkie Keen) and there's also cutiemarks. If you feel lost just look at your butt. But maybe you don't have a butt tattoo yet. In that case it can help little ponies, or their parents, to kinda predict what their destiny/cutiemark is going to be about roughly based on their birth date and family history of affinity. That sounds like it could be useful to predict the names of ponies when they born, which has always been confusing since they seem to fit perfectly with their cutiemark later in life most of the time.

Another possibility is that maybe the pony is having trouble with reconnecting with themselves, not necessarily their cutie mark. Equestria and pony society puts a lot of value on the individual and how they for being themselves are important to their community, how they don't have to be anything else than themselves because they are all already doing their part by just being. There's a lot of focus on identity and how they all have a role to play that no one else can replace even if they might look similar, every one of them are part of something bigger that wouldn't be the same if they were not themselves. One way people in real life use astrology is to help them define themselves. It obviously shouldn't be used to define one's self entirely just like any other thing shouldn't, but it can help you have some thing's clear about yourself and how that is different from others. Start internalizing what you are and what you are not and what can change to get better, how is that a strength or a weakness you should take care of. It gets you closer to know who you are to yourself and others. So yeah, I can see ponies going to astrology consultations to help them find themselves and better define themselves or get help on how to manage some of their personal traits . Not everyone can have a cutiemark crusader at hand to put you on track again, for that there's other types of professionals too that can help with their own method.

Finding love is also a very common use for astrology. This one is self explanatory since it could be similar to how you can find people that you can have better affinity with according to your sign. So for the pony believers it could be used as a method to see if they are compatible with their crush, and since we are in Equestria, magic can come in different forms, so astrology could be another type of magic or have some kind of magical connection that makes it a viable option for certain things.

It would be kinda funny to hear a pony say "i'm a sun Aquarius and Moon Cancer, and according to it i am very sensitive and should be careful for Scorpio types today" or something like that. Although i don't think the constellations have an exact name equivalent, probably. What could be the stories behind them?

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That's what i have for now . In the end nothing is conclusive as for the show stands but it's fun to toy with the ideas of what are presented to us and if we can expand on something for more world building or explore different aspects of our favorite show that's cool. Read you later on another entry of this pony shower thought's.

Do you think Grogar was tossed into the sky so hard he became the Aries constellation? (I couldn't find the original artist but here's the source)