What was Phineas and dog name when they saw it at the museum?

It seems you're referring to the Disney Channel animated series "Phineas and Ferb." However, in the show, Phineas and Ferb never see a dog at a museum. There are many episodes featuring Perry the Platypus, Phineas and Ferb's pet, but he's not a dog and his adventures typically take place outside of museums.

Can you clarify what specific episode or moment you're thinking of?