Today's boosted creature: Banshee
To avoid that always the same creature types are hunted and so eventually exterminated, the Tibian gods came up with the idea of the boosted creature. Every day one creature is chosen which is particularly rewarding for this day. This way adventurers from all over Tibia should be attracted to hunt a larger variety of monsters.
Hunting a boosted creature is extra lucrative. Boosted creatures contain more loot and more experience points. Moreover, they spawn at a faster rate.