Understanding how creatures spawn is essential for a Tamer, be it to train skill, to hunt or to tame highly desired pets. The following paragraphs explain the three standard types of spawns but do not include the champion spawn - that one is outlined in great detail here.

The spawn engine keeps the servers populated with NPCs (Non Player Characters, which can be humans, animals or monsters). To avoid over- or underpopulation, it performs checks every so often to see if the set number of creatures is present in a certain area and replaces what's missing. To create diversity in population of the landmass and the dungeons, NPCs are spawned in three different ways.

Let's take a sheep as an example. For the forest Northeast of Yew the spawn engine is programmed to keep the area populated with a total of X NPCs from a predefined list of types. Among grizzly bears, cougars, cows, goats, hind, timber wolves and others, sheep are one of them. Checks for the number of creatures present are performed at certain intervals and missing ones (killed or removed from the subserver) are replaced. Let's say you kill the sheep: with the next routine check, the spawn engine realizes that the set number of animals in the area is X - 1 and it will queue one creature to spawn. The creature that replaces the missing one can be a grizzly bear, cougar, cow, goat, hind, timber wolf or whatever else is on the list set for the area. This type of spawn can be blocked as long as data of a creature from this spawn is present on the subserver it spawned on, but blocking only effects the overall number of creatures in that area, not the type of creatures present at any given time. Say someone tames all the sheep present in these woods and locks them into their house. While this person reduces the number of creatures that can spawn, the type is not affected. You could kill all the remaining animals and the respawn would be random, including sheep. Other examples for this spawn type are all the forest areas of Britannia, Ice Island and Moonglow.

If you travel to Yew, have a look at the sheep pens there. The spawn engine is set to keep each pen populated with X sheep. If one or more of them are killed or removed from the server, the spawn engine replaces them until the set number of sheep is reached once again. No other creatures will spawn instead. This type of spawn is blocked as soon as a creature from that particular spawn point is tamed and not removed from the subserver at least for a short time, the spawn engine can not detect a difference between tame or wild. If someone takes all these sheep and locks them into their house on the same subserver, no more will spawn until they are moved to another subserver or killed.
This is the type of spawn that causes problems for us tamers. Tame a unicorn in Ilshenar and take it hunting without removing it from the subserver for a moment, a new one will not spawn to replace the missing one until you leave the subserver. This type of spawn is mostly used for dungeons (Destard's dragons and drakes, nightmares in the terrathan keep, blood- and poison elementals in shame etc.), but also for all those creatures you always find in or close to one particular spot (ridable llamas, forest ostards, horses...).

The last stop for sheep and an example on spawn mechanics is the Delucia healer's hut. Sheep spawn in little flocks there, along with groups of goats and cows with one bull. This type of spawn is not permanent and vanishes after a short while (if not killed) to be replaced almost immediately by a fresh set of animals in the same general area. Due to this special characteristic, camp spawns can not be blocked. Even if you tame e.g. a bull from a camp spawn and stable it, it will eventually vanish. This is not a "stable bug". Examples for this spawn type are orc-, ratmen- and lizardmen camps, brigand and gypsy camps or the groups of hinds with a great hart that are very common in T2A.

How long it takes the spawn engine to replace a certain creature can range from instantly to a few minutes, provided it's not blocked by data of the particular creature present on the subserver.