We sometimes accept 'any' with plural or uncountable nouns, but with a singular... it really seems like your own addition. This would be "Nie widzę żadnego byka", rather.
Thanks ! It might be a question of context too (I don't remember if "I do not see a bull" is accepted)...
I'm not a native English speaker, but in my view "seeing someone" is a fixed phrase meaning "to have a meeting with someone". Following that, 'I am not seeing a bull" would mean, 'Nie spotykam się z bykiem' or 'Nie widuję byka'.
But there have been some commotion concerning continuous aspects of English words, that doesn't exist in Polish, so I'm not quite sure how to explain that.