Depending on where you download it from, sometimes the download link doesn't say the version the mod actually is. Make sure ( if your using Forge ) that the Forge version matches the Mod version. Example: Simply Horses 1.7.2 Forge 1.7.2
Here are installation instructions.
Download Simply Horses Mod In Version That You Have With Forge
Make sure you have gone into the forge installer and installed client version.
Go to the Windows start menu and search "%appdata%" open the "Roaming" folder that pops up, then open the ".minecraft" folder and finally open the "mods" folder.
Leave that window open, then go back to Windows start menu and search "downloads"
Find the Mod in the downloads folder
Cut and paste the mod into the mods folder.
Make sure no other mods are in the folder.
Open the vanilla Minecraft launcher and once you get to the screen with the update notes, look in the bottom left corner and click the dropdown arrow next to the username box.
Select "Forge" then "Edit Profile"
Click the dropdown arrow next to "version" select the Forge version that matches the mod.
Save Profile
Click "Play"
When the Minecraft homescreen comes up, click "mods"
If your mod is listed there, then it is up and running!
P.S. Be sure there are no other mods or texture packs running because some aren't compatible.
Hope This Helps!
Gosh this is long!