phrases - "Useless like tits on a log" vs. "tits on a bull" vs. "tits on a turtle" and so forth

I was reading an online book and there was the expression "useless like tits on a log". I googled to find more about this expression and I found a similar one: "useless like tits on a bull". Which one of them is more recognizable in everyday English? Why haven't I been able to find references about the first one?

Edit: I think there is a whole group of expressions like that — "useless like tits on a boar", "useless like tits on a nun", "useless like tits on a turtle" etc. It is really interesting how we can make such an expression.

Answer

There is also a "tit bull," (East Texas), referring to a calf that was never weaned or steered and is still living off mama as an adult.