The Rock NPC is one of the first NPCs encountered in Undertale. They are part of a puzzle involving moving rocks onto pressure plates in order to make spikes go down. Upon the protagonist making contact, the rock asks who told them that they could just "push them around". Rather than pushing them, the protagonist interacts with them and gets them to move on their own. The first time that they are told to move, they go a little forward and then stop. The second time, they move upward. The protagonist tells them that this is the wrong way, and the rock finally understands what they want and moves all the way onto the pressure plate. Once the protagonist goes to cross the spikes, the rock move off. Coming back and correcting them will leave them on the plate until the protagonist crosses.
The protagonist encounters several different Froggit NPCs while in the Ruins. The first Froggit appears after Toriel encourages the protagonist to show other monsters mercy, but does not appear if the protagonist kills a monster (including the earlier scripted Froggit encounter).
Later on, as they near Toriel's Home, they find a room that has a maximum of three Froggits, some only appearing depending on their previous actions. The first tells them about the "X button". If they skipped through dialogue prior to this, then the Froggit will not appear. The second Froggit teaches the protagonist about F4, which puts the game in full-screen mode on the PC version. They assume that this stands for "four frogs", hinting at the hidden fourth frog in the room. The third Froggit teaches the protagonist about sparing and yellow names. The protagonist has the choice of getting rid of the text and changing it to white. If they come back later, the Froggit instead offers the choice of making all of the spareable text pink instead. Between the first and second Froggits inside the back wall lies the secret Froggit. It is ant-sized and waves at the protagonist upon its discovery.