https://stackoverflow.com/a/43880925/1271826

Pets.swift

class PetAnnotation: MKPointAnnotation {
    let type: PetType

    init(_ type: PetType, title: String? = nil, subtitle: String? = nil, latitude: Double, longitude: Double) {
        self.type = type

        super.init()

        self.title = title
        self.subtitle = subtitle
        self.coordinate = CLLocationCoordinate2D(latitude: latitude, longitude: longitude)
    }

    enum PetType {
        case dog
        case cat
        case budgie
    }
}

class PetAnnotationView: MKMarkerAnnotationView {
    override var annotation: (any MKAnnotation)? { didSet { update() } }

    override init(annotation: (any MKAnnotation)?, reuseIdentifier: String?) {
        super.init(annotation: annotation, reuseIdentifier: reuseIdentifier)
        update()
    }
    
    required init?(coder: NSCoder) {
        super.init(coder: coder)
        update()
    }
}

private extension PetAnnotationView {
    func update() {
        guard let annotation = annotation as? PetAnnotation else {
            glyphImage = nil
            return
        }

        switch annotation.type {
        case .dog:    glyphImage = UIImage(systemName: "dog.fill")
        case .cat:    glyphImage = UIImage(systemName: "cat.fill")
        case .budgie: glyphImage = UIImage(systemName: "bird.fill")
        }
    }
}

class ViewController: UIViewController {
    @IBOutlet weak var mapView: MKMapView!

    override func viewDidLoad() {
        super.viewDidLoad()
        mapView.register(PetAnnotationView.self, forAnnotationViewWithReuseIdentifier: MKMapViewDefaultAnnotationViewReuseIdentifier)
        addAnnotations()
    }

    func addAnnotations() {
        let locations = [
            PetAnnotation(.dog,    title: "New York, NY",    latitude: 40.713054, longitude: -74.007228),
            PetAnnotation(.cat,    title: "Los Angeles, CA", latitude: 34.052238, longitude: -118.243344),
            PetAnnotation(.budgie, title: "Chicago, IL",     latitude: 41.883229, longitude: -87.632398)
        ]

        mapView.addAnnotations(locations)
    }
}

Resulting in

Or you can do it on the basis of the annotation type:

class DogAnnotation: MKPointAnnotation {
    init(title: String? = nil, subtitle: String? = nil, latitude: Double, longitude: Double) {
        super.init()

        self.title = title
        self.subtitle = subtitle
        self.coordinate = CLLocationCoordinate2D(latitude: latitude, longitude: longitude)
    }
}

class CatAnnotation: MKPointAnnotation {
    init(title: String? = nil, subtitle: String? = nil, latitude: Double, longitude: Double) {
        super.init()

        self.title = title
        self.subtitle = subtitle
        self.coordinate = CLLocationCoordinate2D(latitude: latitude, longitude: longitude)
    }
}

class BudgieAnnotation: MKPointAnnotation {
    init(title: String? = nil, subtitle: String? = nil, latitude: Double, longitude: Double) {
        super.init()

        self.title = title
        self.subtitle = subtitle
        self.coordinate = CLLocationCoordinate2D(latitude: latitude, longitude: longitude)
    }
}

class AnimalAnnotationView: MKMarkerAnnotationView {
    override var annotation: (any MKAnnotation)? { didSet { update() } }

    override init(annotation: (any MKAnnotation)?, reuseIdentifier: String?) {
        super.init(annotation: annotation, reuseIdentifier: reuseIdentifier)
        update()
    }
    
    required init?(coder: NSCoder) {
        super.init(coder: coder)
        update()
    }
    
    func update() {
        glyphImage = switch annotation {
            case is DogAnnotation:    UIImage(systemName: "dog.fill")
            case is CatAnnotation:    UIImage(systemName: "cat.fill")
            case is BudgieAnnotation: UIImage(systemName: "bird.fill")
            default:                  nil
        }
    }
}

class ViewController: UIViewController {
    @IBOutlet weak var mapView: MKMapView!

    override func viewDidLoad() {
        super.viewDidLoad()
        mapView.register(AnimalAnnotationView.self, forAnnotationViewWithReuseIdentifier: MKMapViewDefaultAnnotationViewReuseIdentifier)
        addAnnotations()
    }

    func addAnnotations() {
        let locations = [
            DogAnnotation(title: "New York, NY",    latitude: 40.713054, longitude: -74.007228),
            CatAnnotation(title: "Los Angeles, CA", latitude: 34.052238, longitude: -118.243344),
            BudgieAnnotation(title: "Chicago, IL",  latitude: 41.883229, longitude: -87.632398)
        ]

        mapView.addAnnotations(locations)
    }
}

Same result.