No, the loot of a creature is determined when it spawns (unless they changed this to be generated on death without telling anyone). In the case of a double loot weekend I assume both sets of loot are generated when the creature spawns. When you have a prey loot bonus, when the creature dies it will roll a second set of loot based on if the prey is activated.
I am not sure if it is still a thing and I couldn't find the old screenshots. But for example, when light settings were low, if a dragon was carrying a life crystal it would glow blue or a dragon lord would glow red if it had a fire sword. If creatures were carrying equipment it actually modified their stats but this has been changed in 2010 according to Mirade.
"The loot a monster will drop does not have an effect on the monsters stats anymore. - What does this mean? Before the update, the equipment dropped as loot sometimes had an effect on the stats of the monster (e.g. slightly increased defence of the monster). The effects of this change are very minor so you should not notice a significant difference."
"A popular rumour says that wearing a Wolf Tooth Chain will increase the chance of dropping a rare item. This has been proven false since the loot of a monster is decided when the monster is spawned, not when killed. An easy way to test this is to kill an Orc Shaman and wait for it to respawn. When it does, kill it immediately. You will notice that it dropped a fresh Dead Snake, whereas waiting for 5 minutes before killing it will yield a snake in the second stage of decay. After an Orc Shaman has existed for 11 minutes, it won't drop any dead snakes at all."