Formal title/honorific for a lawyer

I've never heard of a prefix (assuming you mean prepended honorific) used for lawyers the way Doctor or Professor are used. But it is common for lawyers in the US to use the postpended honorific "Esquire" (as "Esq.") when writing to or about each other. No one ever refers to themselves as Esq, only to others. It's not a legal designation -- anyone can call anyone, or themselves, Esq.

So you'll see letters captioned or addressed to "John Smith Esq." as a courtesy between lawyers, but you won't see letters signed "John Smith Esq."

Here's an article that goes into it a bit.