Wow,

I don't know how representative I am of my fellow OzLanders, but to me:

1. a "merry-go-round" is the thing you call a carousel.

2. a hobby horse is per Merriam Webster's definition (3), i.e. a. a stick having an imitation horse's head at one end that a child pretends to ride b : ROCKING HORSE c : a toy horse suspended by springs from a frame
Primarily definition (a), I'd say. There are no hobby horses on merry-go-rounds. I have never heard a special word for them.

Cheers,
Marty