That is because they were purposely not included in the SRD (System Reference Document) when the publisher designed it. For instance, the D&D 5th Edition OGL SRD does NOT have entries on Displacer Beasts or Mind Flayers, nor do they cover any other feats beyond Grappler. While it is possible that a Compendium could be missing content, especially in the case of newly built Compendiums, it's important to note that an OGL version of a rule system may not cover the entirety of the published rule system it's based from. 'Why isn't there an entry for (Spell, Rule, Monster, Etc.) in here?' The search looks for keywords used in the page title and header tags of an entry. No matter which page of a Compendium you're on, there is a search bar directly above that allows you to look up for more entries at any time. For instance, in the D&D 5th Edition SRD Compendium, the Spells category has subpages for each Class and each one of these pages is a list of spells which have their own dedicated page. Underneath these categories are subpages and in some instances those subpages may be broken down into even further subpages. Depending on the game ruleset in question, the available categories may be different, but what you will likely see are ones such as: Rules, Races, Classes, Items, Equipment, Spells, and Monsters. Once you've selected a Compendium to browse, you'll first have a search bar available followed with an index of categories. This brings you to the Compendium Page where you can choose the Compendium you wish to browse.

You can access the available Compendiums by going to the website's main navigation bar and selecting Tools -> Compendium.

D&d 5e monster compendium