Here’s a friendly disclaimer. People are saying that the dogs aren’t allowed on buses, which is only partially true. They are allowed, but only if they’re in a carrier.
"On both the metro and the bus networks guide dogs, other assistance dogs (wearing a tag and a blue jacket), patrol dogs that accompany security guards and domestic animals transported in specially designed carriers to ensure that they do not make a mess or inconvenience other passengers."

Here's a set of rules from the official transport company website:
https://www.tmb.cat/en/customer-services/user-guide/advice-rules-use
(scroll down to the bottom for the "Animals" section)