Horizon is only the "realtime db" portion of Firebase and it's only for Javascript, not Android or iOS. The mobile app realtime network connection tech is especially tricky to get right without draining a user's battery by doing things like constantly polling the backend. I have not seen anything from RethinkDB along those lines.
> is especially tricky to get right without draining a user's battery by doing things like constantly polling the backend
One simple solution is giving each state a defined name, and keeping a socket open from mobile. If the connection is restarted, each side can name their current state, and the delta can also be easily synced.
Later on, changes can be directly transmitted via the socket.
(I am working on something like that currently myself).