|Sir Gawain battles a "Sea Dragon" - Prince Valiant - Hal Foster|
Megafauna*, or "Giant" animals are nothing to sneeze at. Nature has spent millions of years honing dangerous predators to a fine killing edge, and with a little DM's help, larger versions become a real menace to even the most skilled and well-equipped adventuring parties. A random and unexpected encounter with a giant crocodile at a river crossing can really throw a wrench into your group's plans when they lose a mount or a pack animal, or four, within a matter of a few rounds!
Even animal types that aren't normally predators, become seriously dangerous when you increase their size. Aurochs, giant cattle, can quickly ruin your day when pissed off bull, the size of a large delivery van, decides to charge you. Normally harmless wild weasels that avoid human contact, become aggressive mean-eaters, when you make them the size of large hounds or even a horse. Keep in mind that "giant" anything generally have no normal natural predators in their habitat, because they are too dangerous to attack. Player characters often don't care about this fact and all too often underestimate these "stupid animals".
Giant versions of animals usually use the same successful hunting strategies of their smaller cousins. Hiding under the surface of the water, Laying in wait up in tree limbs, hiding in underbrush, or dugout holes. They can attack from hiding areas instantly, or cover enormous distances with horrific speed. As DM, don't be shy about using these tactics. Giant animals aren't just a collection of statistics to face off against. Don't just throw away some genuine opportunities to add color and bring the wilderness of your campaign to terrifying life. Use megafauna to your fullest advantage.
Because of their increased size, some DMs may rule that very large animals automatically gain initiative over opponents not attacking with reach weapons, such as polearms, or ranged weapons. Furthermore, they may rule that attacks using close combat weapons (most normal hand-held weapons) will initiate a free counter attack by the larger opponent. Even if attackers use spears and other polearms, some DMs may rule that the animal may get a free counter attack against the weapon, attacking an AC of 5 (plus any Dexterity bonus), to force the attacker to roll a Save, or lose their weapon to a break or disarm (roll 1d6; odds, the weapon breaks.. evens, it is torn from the hands of the wielder).
* - Megafauna, (from Greek μέγας megas "large" and New Latin fauna "animal life") are large or giant animals. - Wikipedia