if there was a comma"come on, off now" I would have understood it!
The problem is that it's correct without the comma and doesn't mean quite the same thing as "Come on, [get] off now".
"Come on off..." can be used as a phrase in the same way that game show presenters, when they invite someone to come out of the audience and join in with the game on stage, they say "John - Come on down!" It would be equally understandable as "Come down [to the front]" but it just wouldn't sound the same.
In the same way, the speaker could have said "Come off now" meaning "Get off the horse now"