I wonder if it's correct "ο θείος μου έχει μια άλογο"
No, I don't think so. Horse (το άλογο) is neuter, so the indefinite article is ένα. If the uncle had a cat (η γάτα) which is a female noun, then you would use the indefinite article μια.
Thank you very much
Why is this in a lesson about time?
If you found this one on Strengthen, it's not unusual. All kinds of sentences from all kinds of lessons could pop up there. :P
That's the thing, I didn't. I was doing the lesson for the first time. (It was lesson 7 or 8 or so under "Dates and Time".)
Duo has the habit of popping up irrelevant exercises even in first time lessons. The sentence above belongs to the family skill. ;)
I had it right and it gives me error twice