I'm considering buying a Delcatty, Arcanine or Ninetales mount.
Wanted to know how much it would cost, thank you

There is a quest for an free Arcanine mount actually, which you can check out here: https://prowiki.info/index.php?title=Arcanine_Mount_(quest)

Mounts from the coin shop cost 50 coins for the normal version and 100 coins for their shiny version (coins are a currency you can purchase with real money).

If you want to buy them for ingame-currency from other players, you would have to pay around 400-450k pokedollar to get a Coin Capsule, which you get 100 Coins upon using it.

You can try to get the mounts themselves a little bit cheaper on trade-chat, but that may take a little longer.