The skeleton is one of the most iconic fantasy creatures, and it's been with D&D since the earliest of days. The imagery of skeletons have been part of human culture for as long as we know, and its association with death is a natural one. They've also been an important part of popular culture the last century, and the animated skeletons of Dungeons and Dragons probably owe a lot to Ray Harryhausen's fantastic stop motion animated skeletal warriors from the 1963 movie "Jason and the Argonauts".

Giant Spiders are a common reoccurring threat in fantasy literature, featuring quite often in the works of Tolkien, including the spiders in Mirkwood from the Hobbit, Shelob from Lord of the Rings and the fearsome Ungoliant described in The Silmarillion.

As such there is no surprise these creatures always have been an integral part of Dungeons and Dragons, appearing in many shapes and sizes from the earliest editions.

Wolf Spiders are hunting spiders, rather than webbing spiders, so quite why the posses web sense and web walk abilities are a bit of a mystery....

The next monster up is the Blink Dog - the first (lawful) good creature covered here, and as such probably more likely to be encountered as an ally than an enemy. The Blink Dog has been with D&D since it's early days, but unlike a lot of other monsters, the Blink Dog seems to be an original creation for the game, having no basis in mythology or folklore.

For some reason Blink Dogs have always been the sworn enemy of Displacer Beasts.

As with a lot of giant animals and insects, the giant centipede has real lift counterparts - like the Arthropleura, huge millipedes that could grow to about 9 feet in length. In D&D terms they're also quite ancient, appearing in the original Monster Manual - though the ones described there are no more than a foot in length.

Animated objects, here in the form of a Flying Sword, have probably been with the game since the start (and not just as golems and undead). In original Player's Handbook, the spell Animate Object was part of a cleric's repertoire (as a 6th-level spell), but in 5th edition, the spell is no longer available for clerics, instead being part of the bard, sorcerer and wizard's spell lists.

The Axe Beak appeared in the original Monster Manual, and was one of the first monsters you'd see when you opened the book. They're inspired by prehistoric, flightless birds (related to today's ostriches, emus and rheas), and the massive beak might evoke images of the Gastornis, that lived some 56-45 million years ago. Other gigantic flightless birds have existed, notably on Madagascar, where the Elephant Birds lived, probably up until the 17th or 18th century, the largest of which grew up to 10 feet tall.

Today's monster is not of the fantastic sort, but a mastiff - a large guard dog. The drawing is loosely based on a mix between a Bull Masiff and an English Mastiff. Not a timid little lapdog, these magnificent beasts normally weigh between 70 and 90 kg (150 to 200 lbs.), with a shoulder height of roughly 75 cm (30 inches). Their use as guard and war dogs might go back as far as Roman or even Assyrian times.

Back in 3rd edition the idea of halfling dog riders were quite common (they even had their own prestige class). While it was never really specified what kind of dog these halfling cavaliers used, it would probably have been a fairly big breed, like the masitff or similar.