Without including event or boss creatures, according to my source TibiaWiki, the creatures with the most hit points in Tibia is (only listing top 5):
Infernal Demon
Some other creatures seem to be 28,000 HP, please see my source TibiaWiki for the full list :)
Top 5
1. Sight of Surrender - 28.000 hp
2. Icecold Book - 21.000 hp
3. Juggernaut/Cursed Book- 20.000 hp
4. Energetic Book - 18.500 hp
5. Burning Book - 18.000 hp
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