My hippodrome simply won't fetch horses for the race. There were 3 horse farms available with 8 horses each, but it didn't bother fetching them. I build another horse farm right next door and connected it to the road and bridge to the hippodrome loop, but still nothing. All it needs to run at this point are the horses. What am I doing wrong?

Did you try right clicking on the hippodrome, it should tell you why it is not active and how many horses and workers are there.
Can you post that if you are still unable to get it working, also a bigger picture which has the whole loop might help.

Bump..the same thing is happening for me as well...any insight on how to fix this would be appreciated.

In my case, the only reason the hippodrome wasn't working is because the horses were not yet on site. I was tired of waiting, so I logged off. When I came back to the game, the horse trainer instantly went out to get the horses next door! Maybe logging off the game and starting again could work for you, too. Sorry I don't have a better answer.

I once had a similar problem where the merchants wouldn't fetch things for the agora.
The problem was that I had a road running just between two buildings, an "island" not connected to the main road system.. once I destroyed this things started working again.