D&D has a wide variety of dangerous monsters for a party of adventurers to encounter anywhere their journey takes them. Flutes Loot discusses the CR difficulty rating system, and the ups and downs of working with it.
The familiar used to be one of the only ways for a player to have a creature of some sort at their side. Dump Stat Adventures goes through a brief history of familiars in D&D, and reviews various creature options for your own familiars.
Surprise as a mechanic is confusing yet straightforward and is incredibly underused as a tactic in combat. Dump Stat Adventures goes in-depth on surprise and how to use it.