The short answer is, you cannot call your horse in Skyrim because there is no function to support the action. However, there are some ways around it.
The quickest option is to simply fast-travel to an outdoor location and your horse will automatically appear.
Alternatively, if you own the Hearthfire DLC, and have purchased a horse, you will be able to locate the steed at your stable.
Other games, like Assassin’s Creed, have an in-built whistling mechanic that will trigger your steed to gallop towards you. Sadly, Skyrim does not support those joyous moments, but at least it allows you to transform into a werewolf.
Skyrim horses that have a pseudo-calling system
After completing the Dark Brotherhood quest, promoting you to a prestigious member of said Brotherhood, the exclusive steed Shadowmere will be available to you.
Similar to the fast-travel method, the mare will be available to mount at any outside location, providing you fast-travel there first.
If you own the Dawnguard expansion, you will be able to complete the quest to find Arvak’s skull in the Soul Cairn. After completion, you will be taught the spell to summon Arvak.
You will then be able to manifest the spirit of this skeletal steed anywhere in the open-world.
A final alternative to calling your horse is to download the ‘Call Your Horse’ mod from Nexus Mods.
Created by Otellino, the Call Your Horse mod is an effective tool that you can use to call your last-used steed, providing you own one and are located outside.
Similar to the Assassin’s Creed mechanic, the mod will let you whistle for your horse’s attention. If your horse is relatively close to you, it will trot towards you, otherwise, it will spawn off-screen and gallop to your location.
Additionally, if you do not mount it straight away, it will follow you for a short time before opting to plod around the vicinity. If you leave its area, it will return to your stable.
Once you download the mod, a Lesser Power will be added to your character, which has no cool down period and costs no Magicka.