best sensor for detecting object and approaching them

this sensor will be used on a robot arm

Minimal information will get minimal response.

Weedpharma

Btw, is it possible to have such sensor and detect eventual nearby animals in wilderness?

Yes, of course.

I should've said that the sensor would be on a wearable device.
If the sensor is on a watch, can it detect other moving animals/humans?

What principal of detection do you have in mind. I am not aware of a sensor that is not at a fixed point that can detect humans of animals.

Weedpharma

a PIR sensor, perhaps?

A PIR is basically directional (I know they have lenses and can cover an angle). You would need a few around you and moving around would probably effect their operation.

Weedpharma