In the first video he remembers some of the statements made by Valve in 1997-1998, prior to the release of Half-Life 1, where they told about their plans to add "alien breeding" in the game, which would repopulate areas visited by the player so he would meet new monsters if he would decide to go few maps back. Another feature they mentioned in interviews was dynamic moss groth on walls, which, according to their idea, we were to be able to watch in real time. As we all know both of the features were never fully developed and were cut from the game. What's interesting is that for some reason Gabe Newell was especially tied to the moss groth feature and mentioned that he'd like to see it in game even in 2003, when he was talking about Half-Life 2.
The second video is dedicated to another monster cut from Half-Life 1 - Kingpin. This monster had its own model but only 3 animations exist on the version we have in our hands. According to few sketches and pieces of information that can be found in Uncorrected Proof version of Raising the Bar, we were to meet this monster at least twice in the cource of the game: in a huge hangar during "Surface Tension" chapter, where he was to be tormented by soldiers and we were able to set him free and make him fight them, and near the end of the game, at Xen, where these monsters were to guard the entrance of a cave.