I don't remember there being any.
There are flat horse counters which work with them in something from GURPS (the blank map pack?), but GURPS horses are 3-hex figures while TFT horses are 2-hex figures.
I think flat counters work well for mounts since you can very easily put a standing figure on a flat horse counter to indicate it is mounted.
So if there are no official Cardboard Hero horses (?), I would just make my own slew of 2-hex top-down horse counters, e.g. basing off of something like:
https://www.google.com/search?q=top-...w=1920&bih=925