A lot of Pokemon have alternate forms, regional variants, Mega Evolutions, or Gigantamax forms. The detail page shows all of them with images and names. If a Pokemon has variants, they're all in one place.
Step 1
Open the Pokemon's detail page from the Pokedex.
Step 2
Go to the General tab and scroll to Alternate Forms. This section only shows up for Pokemon that actually have variants. No forms, no section.
Step 3
Each form has its sprite and full name. Pikachu's page, for example, shows Rock Star, Belle, Pop Star, PhD, Libre, Cosplay, a bunch of Cap variants, Starter, and Gigantamax.
Step 4
Regional forms like Alolan, Galarian, Hisuian, and Paldean show up here too when they exist. Each one has its own sprite so you can see how the regional version looks different.
Step 5
Right now the site shows images and names for all forms. Detailed data for each individual form is coming in a future update.