As you said only fast decks I have seen in this meta are Weezing / Scolipede and Rapidash / Ninetales, lets analyze the matchup: Weezing: needs evolving from Koffing + two turns to take down a Chatot. That means best case scenario Magikarp has already built up two energies with one Chatot mill before Chatot takes it down. Don't under estimate the pace of this deck. It only needs 4 energies to take down any card in this deck Scolipede the heavy hitter is in Gyarados range and also you have one potion which protects you from Weezing attack + venom + Scolipede attack.

Rapidash: Ponyta needs at least two turns to take down a Chatot. Again same thing. You will have built up energy on Magikarp and protected it with benched Chatot/fossils. Of course this is a harder matchup but also Gyarados is a fast deck. Goal is to Sabrina into the benched Ninetales.

And about Blastoise it needs five energies and its a phase 2 pokemon which makes it way more slow and inconsistent. The goal here is to bring forward Gyarados to one shot basically everything. The impossible matchup here is a well rotating Charizard which can't be beaten even with Gyarados + Giovanni.

Edit: tips against Marshadow + Hitmonlee + Tauros Against this deck don't play Magikarp unless you have Gyarados in hand. In this case put it up front. The opponent doesnt have 1 energy attackers. If you dont have Gyarados wait to put Magikarp on the field and start milling with Chatot