"My uncle has a horse."
Translation:Ο θείος μου έχει ένα 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.
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".)