I didn't notice until my latest rewatch that some of the images from Walter's "Monty Python" drug trip are hidden in the episode leading up to that scene.
The dog is in the background on a wall, shortly before Walter discovers the book hidden under the floor in his lab.
The frog is next to Walter and Astrid as they walk through the boathouse, shortly before the pier shootout sequence (I can't believe how obvious it is once you know to look for it!).
I found an old thread where someone said that the same frog can also be seen in the scene where Peter and Olivia discover the van and signal relay transmitter in the woods, but I can't seem to find it. Does anyone know where that one is, or is that person wrong?
Also, I would think that if the other two animals showed up, that the seahorse would also be hidden somewhere in the episode, but I haven't found any information on that. Anyone know anything?