When did Snape learn Sirius was an animagus?

Question

In Harry Potter and the Prisoner of Azkaban when they are learning the truth about Sirius, they all hear a creak behind Lupin and the door opens by itself. We go on to learn that the Marauders are animagi. Then Snape reveals himself.

Was he listening under the invisibility cloak the whole time since they heard the creak? If so, Snape would know Sirius is an animagus. Then why was he shocked at the end of Harry Potter and the Goblet of Fire in the hospital wing when Sirius transformed from dog to man?

Answer 1

Snape wasn't particularly shocked.

Snape had not yelled or jumped backwards, but the look on his face was one of mingled fury and horror.

If you're asking why he didn't recognize Sirius's dog form, he'd never seen it. He had no reason to expect Sirius to be at Hogwarts, so it wouldn't be likely to occur to him that this might be him.

Snape was angry, because he hates Sirius, and horrified at the prospect of having to work with him, and perhaps still of the opinion that Sirius was working for Voldemort. He wasn't shocked to learn that Sirius was an animagus; as you say, he already knew.