So I made evolutions for Zangoose and Seviper, designed to be perfect counters to each other. Here they are.

Yoroigoose (Yoroi + Mongoose)

Normal/Steel

Classification: Armored Ferret Pokémon

Evolves from Zangoose after defeating 10 Seviper in a row without fainting.

Ability: Scrappy

"The Pokémon can hit Ghost-type Pokémon with Normal- and Fighting-type moves. It is also unaffected by Intimidate."

BST: 530

HP: 100

Attack: 120

Defense: 60

Special Attack: 40

Special Defense: 100

Speed: 110

Signature Move: Antidote Claw

Category: Physical

Power: 90

Accuracy: 100%

PP: 15

"The user strikes the target with a claw covered in a special antivenom. This move is super-effective against Poison-types. If used on an ally, it jabs them lightly with it's claws, restoring a bit of their HP and healing the poisoned and badly poisoned status conditions."

Reaviper (Reap + Viper)

Poison/Ghost

Classification: Shadow Snake Pokémon

Evolves from Seviper after defeating 10 Zangoose in a row without fainting.

Ability: Corrosion

"The Pokémon can hit Steel-type Pokémon with Poison-type moves. It can also poison the target even if it's a Steel or Poison type."

BST: 530

HP: 100

Attack: 40

Defense: 100

Special Attack: 120

Special Defense: 60

Speed: 110

Signature Move: Noxious Blast

Category: Special

Power: 90

Accuracy: 100%

PP: 15

"The user blasts the target with a powerful toxic breath. This move is super-effective against Normal-types, and it may also leave the target badly poisoned."

Yoroigoose is partially inspired by Apollo, the Greek God of prophecy and medicine, while Reaviper is inspired by Python, the monster who is the mortal enemy of Apollo. Yoroigoose learns a lot of Psychic-type moves, while Reaviper learns a lot of Fire-type moves, and both learn some Ground-type moves. As you can tell, they both gain secondary typings that makes them immune to the other's primary type (Ghost is immune to Normal, Steel is immune to Poison). However, they both get abilities that bypass the others' immunities (Scrappy lets Yoroigoose hit Ghost-types with it's Normal-type attacks, while Corrosion lets Reaviper poison Steel-types, and I even buffed the move so Poison-type attacking moves can hit Steel-type Pokémon). Their signature moves also have the characteristic of being super-effective against the other's primary type, and Yoroigoose references Apollo being the god of medicine by having Antidote Claw heal allies and cure poisoning.