The creatures of the Dominion have exactly that. Dominion over the tropical overworld and much of the underground, stretching even into the temperate areas of most continents. They may be adapted directly for heat or underground life, or simply capable of it and transition from one to the other at their own convenience. They all share a territorial nature, whether they are willing to get along with others of their own species or not, and for this reason tend to colonize any land that no other, stronger creatures have claim to. Almost all habitats possible for them are now under their 'dominion' due to this, and they generally only clash with other creatures at the fringes of these habitats.
Due to their territorial nature, they are usually skilled hunters and moderately destructive, either through their direct power, or their ability to track their enemies. Even the herbivores amongst the Dominion are capable of dealing with enemies by calling the herd to overwhelm them, and it is this freedom and control over resources that allows them to hold their position. They show the least overall cooperation between themselves, though, with only a few creatures willing to directly assist those outside their race, or sometimes, even those within it. Despite this, when threatened, they become a formidable force purely because if their territory is invaded by foreign beings, they are collectively more concerned with eliminating those new enemies than the rivals they are already used to.
In the Aztotl language (also used by most nomadic Marauder tribes), the term for the creatures in this 'class' translates roughly to "(emphasis expletive deleted) awesome thing insane to ride". The most appropriate translation therefore for us is therefore the memetic 'Epic Mount'. These range in size from twice that of a 'normal human' to towering giants that seldom move due to the relative difficulty they would have in supporting their own weight, were it not for their exceedingly dense skeletal and exoskeletal structures. Some even resemble what we would call giants or golems, but are not quite as colossal. Though they are often slow moving and therefore not able to attack very powerfully, many are extremely difficult to damage, making them good at holding their ground, though their lack of speed prevents them from being able to effectively block attacks on other creatures or locations.
They are strongly attuned to elemental energies and in some cases, particularly absorbing of these. Heat and Light, and sometimes even gravity, are absorbed directly into their bodies and used to power their motion, and they can therefore be found in many places. Other creatures often gravitate towards one known as the "Gaia Rumble" just to cool off on hot days, as its ability to absorb heat and other elemental energy makes the environment around it mild and pleasant. It is also surmised that extremely aged, large "Gaia Rumble" creatures become what translates as "Archaic Rumble", creatures so heavily armored and absorptive that most elemental 'magic' and attacks have no effect whatsoever regardless of their source, and the area around these giant ones is an almost dead 'calm zone'... until they move.
Because they do not really need to fight for territory, most of the creatures dubbed Epic Mount go about their business in the Dominion lands unmolested, and are often inclined to actually fight alongside others. Whether this is because they get to absorb energy from the fighting, protect their food supply, or are simply benevolent, seems to vary.
For those who are used to snakes of generally 'manageable' sizes, the massive Grand Serpents of the Dominion are terrifying to encounter. Practically infused with the power to manipulate Gravity, their powerful bodies defy the limitations of size. In almost all cases, these great snakes are constrictors. Those bearing the names of venomous snakes in our world are named only due to resemblance in their patterns and body shape. Moving silently, they often take their enemies by surprise in one way or another, resulting in swift and often unavoidable destruction. Their control over their weight allows them to slither easily through some of the most difficult terrain, and because of this they will often chase their prey in the direction of rocky, uneven ground, or ambush them from trees after driving them into deeper parts of a forest or jungle.
Even the smallest of these creatures, mostly known as Vibra due to its strange ability to change its coloration by shivering its scales in particular way, is large and powerful enough to easily crush many adult humans without needing to engage in any prior 'combat'. By contrast, the massive Black Carpet, which some surmise may simply be a full grown, or perhaps mutated overgrown form of Vibra, has such powerful coils that nothing at all survives its constriction, save perhaps the Archaic Rumble. Fortunately for those who travel in groups, once a Grand Serpent has crushed its prey, they are exhausted and not normally quite so formidable, giving the remaining members of the group the chance to dispatch it. Still, it often requires some magic or other effect to escape without at least one tragic loss, as the Serpents are both fearless and merciless in their hunting. Being cold blooded, though, they are often weak against Ice, and this often serves as one of the best possible defenses against them, especially in terms of elemental magic. Ice creatures themselves do not usually fare much better than their counterparts though, since their often-fragile bodies are easy for the Serpents to shatter.
Grand Serpents are intelligent enough to easily discern friend from foe, and in fact make 'friends' easily, due in part to the fact that they are simply not very territorial. This allows them to defend their territory much more completely from other creatures, and a large group of them can quickly become a serious threat, since they can easily ambush larger groups of prey at a time, removing the risk of being the victims of any vengeance from Marauder groups who have lost a comrade, and so on.
Evolutionary biologists would happily spend time searching for a common ancestor between the Phoenix creatures of the Regency, and these grounded fire lizards that are best described as Salamanders, in the mythological sense. Salamanders have high physical heat outputs, facilitated by special secretions from their scales that allow them to produce actual flames. Their scales, in contrast, are flame retardant, allowing them to survive this usually. Other creatures, particularly smaller ones that get in their way or are their targets, are not often that lucky, and many Salamanders grow to be bigger than the average human in length and weight. Salamanders themselves are quick moving creatures with eyes adapted for hunting and tracking small, fast targets, allowing bigger ones to attack creatures that are not even exhausted, sometimes. Bursts of flame and acceleration make this even easier for them, though because of this, there are various drawbacks to the creatures use of their full power, ranging from need for extra energy, to uncontrolled burning of their surroundings and even allies.
Salamanders get their names from the colors of the flames that flicker on their bodies, and their general temperament. Each subspecies has a different hue of flames, presumably due to their diet and various chemical ions in the flammable liquids they produce. These colors range considerably, from the blue-green flames of the sometimes self-destructive "Turquoise Temper" to the dark red, somewhat cooler flames of the "Burgundy Wail". Salamanders themselves do not often congregate into large groups or work together, except when living near water. Surprisingly, they are all good shallow-water swimmers, partly due to the increased buoyancy, but cannot stay in the water for long periods as it cools them too quickly, leaving them lethargic and likely to drown. Sometimes Salamanders are saved from this fate by other creatures, or allowed to ride on the backs of larger waders when enjoying a dip. This is important to them, since, despite their ability to retain most of their body fluids, they dehydrate quickly under certain particularly arid conditions or if they are injured. In fact, it is generally due to being helped by other creatures when injured that Salamanders form bonds resembling friendships at all.
Of all the creatures of the Dominion, Salamanders are perhaps the most territorial, and also the most indiscriminately destructive, a bit of a bad combination. Their streams or bursts of flame often target anything their vision can perceive as moving and possibly a meal, even if half the time it is nothing of the sort, forcing weaker creatures to avoid associating with them in combat.