I did the 2 fast quests for Riften & Solitude Stable Owners & have 4 free horses to use. However, the Unsaddled Solitude one went missing (I probably rode him somewhere and then switch horse - I think) and it has been well over 10 days now, if not 30 days already, and the horse hasn't returned.

I've witnessed these horses being killed by spiders, dragons, etc. before and they respawn after few days. I left a Saddled Solitude Horse in Riften and it returns to Solitude Stables after 2 days. But the one I'm talking about (Unsaddled Solitude Horse) has been MIA for weeks. What can I do to find this horse? Is it possible to console command myself next to this horse to see where it is?

You can try this:

prid 000ce66fmoveto player

or (in case the horse would be dead)

prid 000ce66fresurrectmoveto player

Not sure if it'll work though.

Third option:

player.placeatme 00109e3e 1

This command will create one occurrence of the Solitude stables horses.