Learn Spanish (Latin America): El perro se quedó inmóvil por el miedo.

Quedarse is specifically used for changes that express a resulting state, especially those involving a loss, a physical incapacity, or a sudden lasting consequence (like going blind, deaf, or motionless). Ponerse is usually for temporary emotional or physical changes, and volverse is for deeper changes in personality.

Could I just say quedó inmóvil without the se?

It sounds much less natural. While the non-reflexive quedar can mean to be located or to have something left over, the pronominal form quedarse is the specific verb of change used to mean ending up in a certain state. The se is required to emphasize the transition into that physical state.

Why do we use por instead of para before el miedo?

Por is used here because it indicates the cause or reason for the action. The dog stayed motionless because of fear. Para would indicate a purpose or destination, which doesn't fit in this context.

Why is there an el before miedo when the English translation just says 'out of fear'?

In Spanish, abstract nouns and general concepts like fear, love, or time generally require a definite article. While English drops the article for general ideas, Spanish needs that el to grammatically say 'because of the fear.'