They appear every 4 hours starting from the server save, so you can find they them at 10h, 14h, 18h, 22h 22h, and 02h (based on the time CipSoft uses on their website). If you didn't found couldn't find any Buffalos, someone could have killed had to kill them and they didn't appear again.
Source: https://tibia.fandom.com/wiki/Water_Buffalo