Hi and welcome🙂
I havent used the 90D but it's very similar spec'd brother, the M6mk2.You don't say how experienced you are so i'll give some basic hints in case you're a beginner.
If the animals are in good light and not moving much you are free to choose almost any setting.Usually one would set the ISO for best quality and use Av mode to set aperture for the look you want to achieve.Eye focus recognition can be useful.I'm pretty sure that camera will choose the eye of the closest animal, or if a large group one of the animals in the center. In dim light a tripod may be necessary.
If the animals are moving,things get more difficult. You'll need to choose a shutter speed high enough to freeze the action.Focus tracking will struggle if the subject is coming towards you.Increasing aperture (if possible) to give a deeper depth of field might help with that
