Checks and Counters:
So beating Ho-Oh is easier said than done. For starters, its prime Fire STAB move, Sacred Fire, causes burns to targets 50% of the time, a statistic that most physical attackers hate to see. Even the Fire types in Ubers like Primal Groudon have a tough time against Ho-Oh due to its additional access to Earthquake. Along with this, if you send in something Ho-Oh dislikes seeing, like Primal Kyogre, it can switch out and recover 25% of its health. Unlike most "bird" Pokémon, it has access to two forms of reliable recovery in Roost and Recover. So what's the best way to counter it? Well Ho-Oh hates Stealth Rocks due to its losing 50% every time Stealth Rocks is active and it switches in. Chances are that if you see Ho-Oh on a team, something else on the team will have a way to counter entry hazards, like a Rapid Spin or Defog user, or a Pokémon with Magic Bounce. The two Primal Pokemon are a good way to counter Ho-Oh, because not only do they counter Ho-Oh with Origin Pulse/Stone Edge, but they can also deal with Rapid Spinners like Excadrill and Mega Diancie with Origin Pulse/Precipice Blades. Primal Groudon can better handle Ho-Oh due to the bird's lower physical defense, Groudon's high physical defense, and immunity to burns, meaning it can safely switch in, and it gets access to Stealth Rocks. Primal Kyogre deals heaps even with Ho-Oh's larger SpDef due to rain boosted STAB Origin Pulses. Obviously, any Pokémon that can counter Ho-Oh also needs to be wary of Toxic. If you can switch it in on a predicted Toxic, Mega Diancie can stop Ho-Oh cold. However, it's not ideal to switch it in most of the time due to Sacred Fire and Earthquake, and it can't Magic Bounce Toxic if it hasn't Mega Evolved, though if it hasn't Mega Evolved yet, it can eat an Earthquake better, Mega Evolve, and then maim Ho-Oh with Diamond Storm. The bad thing about Ho-Oh is that it's super hard to switch into with its good coverage and ability to burn opponents. There's a decent amount of mons that can better handle it with a free switch-in though, like Zekrom, bulky Mega Salamence with Facade and Refresh, Tyrannitar, Arceus Water, Rock, and Stone Edge users like Mega Lucario, Arceus Fairy, and Zygarde-C.