My first horse was an ancient (28 year old), retired mare I inherited from my riding instructor when she moved barns. She was my favorite horse, and I was equal parts delighted I'd get to see her everyday and nervous I wouldn't be able to properly take care of her. The first couple of days (weeks, months, lifetime), I pestered my Dad with questions about anything and everything.
"Dad, her ears are drooping. Is that normal?" - She was just relaxed.
"Dad, she sneezed/coughed. Is she okay?" - That's what happens when they're eating hay and dust gets in their nose. While a little dust is fine, too much can cause respiratory problems, such as HEAVES.
"Hey Dad! How can I tell if she's colicing? I think she's colicing right now!" - She wasn't. She was scratching an itch. She never colic a day in her life and was offended I'd even entertain the thought. Never-the-less, knowing the signs and how to PREVENT COLIC is important.
"Dad, what are these weird bumps? Does she have cancer?" - They were bug bites. Use FLY PREVENTION.
"Dad, her poop is weird. It's green! That is not normal! Can you come look at it?" - That's what happens when horses eat a lot of fresh forages. However, ANALYZING YOUR HORSE'S MANURE can be a great health indicator.
My poor, patient Dad answered every question, and eventually I was able to establish a baseline of what is normal and abnormal. While it is funny (and a little embarrassing) to look back on this time, never-the-less, establishing a "normal", or baseline, is an important step when becoming a horse owner. It not only helps you detect signs of disease early, but also helps your vet form a diagnosis in critical or emergency situations.
Arizona Cooperative Extension has a great resource any savvy horse owner can use to establish a baseline on not only behavior and eating patterns, but also vital signs, gum color, and more (link below).
Knowing What is Normal for Your Horse