8. The Cleric Beast - Bloodborne
Given that for most players it's the first boss they fight, it's likely pretty hugely surprising that a good portion of first-time plays may have missed the Cleric Beast entirely.
After all, while it's one the two bosses in the first area you're put in, it's an entirely optional one, since there is no path revealed after beating the bony bad boy. As such, if you find the other route first - beating the blood-addled half-beast Father Gascoigne - you're likely to not search to area further, as you've places to go and monsters to be killed by.
This is especially unfortunate because killing the Cleric Beast gives you the Sword Hunter badge, which unlocks a series of items in the shop you find in the Hunter's Dream. Amongst a bunch of useful weapons, this also unlocks the Hunter Chief Emblem, which unlocks the door to Vicar Amelia - who is not an optional boss.
So, how would a player still not find the Cleric Beast? Well, the Hunter Chief Emblem is not needed to get to Amelia - but it is the easier way. Instead, you need to find a secret route that leads you to either a hidden area, or to the Vicar boss fight.
Hopefully at this point you've googled enough to realise you've missed a boss though, because this is one you don't want to miss.