What is the prefix to beside?

There isn't a single, commonly used prefix for "beside". "Beside" itself is a preposition, meaning "at the side of" or "next to."

However, there are some related words with prefixes that might be useful:

* Besides: This word uses the prefix "be" to indicate "in addition to" or "apart from."

* By: This word has a root related to "beside" but with a different meaning, typically indicating nearness or proximity.

Ultimately, the best word choice depends on the specific context and the intended meaning.