Yes, it is possible for a monster to turn fiendish. The basic mechanics are:
1) There only can be 3 fiendish monsters on all server at the same time.
2) If a fiendish monster is killed, a new one will born with a 5-min delay.
3) If the fiendish monster isn't killed for 1h, it loses its status as a fiendish creature and after 5 min a new one'll receive this status
Because of the extreme rarity, it isn' probable that you'll find a monster in your hunt, but it may happen.
Sources: Tibia website and forum (FAQ) + tibia.fandom.com