Don't know if this is known, but it seems that Meeko will not appear if you have a current companion.

Had Barbas with me (for his quest) and it seemed like Meeko didn't appear due to having a current follower.

I wanted Meeko as my trusty companion

Not true, at least insofar as human companions are concerned. I picked up Meeko when Lydia was faithfully tagging along, carrying my burdens.
Alas, the dog died in a nasty, trap-filled dungeon. He succumbed to his wounds before I could cast Healing Hands on him.
Couldn't be arsed to reload a previous save (what with Meeko & Lydia constantly dying in that dungeon) so now I'm adventuring without him. Can't say I really miss him...
However I suspect that with Barbas your observation holds. You could pick ip Meeko later I suppose, once you're done with Barbas and his quest.