Its actually not that bad of a card as people are making it out to be.
Because of the presence of Charizard Ar, any fire pokemon will help. This is a fire pokemon and it has a low energy attack. For just 1 fire energy you can burn your opponent. It also has free retreat, which lets your burn, hopefully take 1 hit, and then hide away on the bench and help charizard.
Burn is helpful because many pokemon do not burn. Yes Magmortar and Blaziekn pt, can burn on the bench, but who wants a pokemon with high energy requirements, or a stage 2 instead of a stage 1.
Another reason Rapidash is good is because other rapidash's are good. You could run 2 or 3 ponytas and then run 3 rapidashs (one of each arceus and one sf). This gives you some sniping ability and some sp tech ability, along with a burn and retreat option.
Rapidash's are very flexible and pretty decent.
This card isnt the best, but the burn option does work in a charizard deck. 4.5/10