Learn Spanish (Latin America): En el safari, vi al rinoceronte y lo escuché respirar. - On the safari, I saw the rhinoceros and heard him breathing.

When using verbs of physical perception like escuchar (to hear) or ver (to see), Spanish frequently uses the infinitive to describe the action you perceive, even when English uses an "-ing" word. You could also say lo escuché respirando, but the infinitive respirar is extremely common and sounds very natural here.

Why is it al rinoceronte and not just el rinoceronte?

This is the "personal a" combined with el to form al. You likely learned to use the personal a for specific people, but we also use it for specific, identified animals. Since you are talking about a specific rhinoceros you focused on during the safari, it gets the personal a.

Why is the pronoun lo placed before escuché instead of attached to respirar?

Lo is the direct object pronoun representing the rhinoceros. In structures with a perception verb and an infinitive, the object pronoun almost always goes right before the conjugated verb (lo escuché respirar). While it is technically possible to attach it to the end of the infinitive (escuché respirarlo), placing it before the main verb is much more natural and standard.

Could I say lo oí respirar instead of lo escuché respirar?