Learning how pets spawn is very important to a tamer. Their livelihood depends on it. Once you understand the spawns, you can use them to your advantage. There are 3 types of spawns in UO. They are the camp spawn, pool spawn, and single spawn. There are advantages and uses to each spawn type.
What is the "spawn queue"? The server knows exactly how many and what kind of creatures should spawn in a particular location. When the server sees that the appropriate number and type of creature is found on the server, it stops producing monsters or animals. If any particular creature dies or leaves the server, the server will pick up on this and add it to its "queue" of creatures to spawn. No one knows how often the server will check for missing creatures or how long it takes a creature to spawn once it is added to the queue.
You kill a lich lord (single spawn). The server sees that a lich lord is missing and adds it to the spawn queue. A short time later, a new lich lord spawns.
You kill a cow (camp spawn) outside of Delucia. The server doesn't care about your cow. The rest of the members of its camp spawn will vanish shortly and a new camp spawn will form eventually. Player actions do not effect the camp spawn at all.
You tame a white wyrm (single spawn). If your tame wyrm does not leave the server, a new one will not get added to the spawn queue and a new one will not spawn. Once you remove your tamed white wyrm off of the server it spawned from, it is now removed from the spawn pool and it gets a new "spawn ID". You can watch wyrms respawn 'til your heart's content once you return your newly tamed wyrm back to the server it spawned from.
You kill a goat (pool spawn) outside of Moonglow. The server sees that there is a creature missing once the goat dies. The server adds a random creature to its queue from the pool spawn list for that area. A short time later, the random creature spawns.