Glados has a habit of ‘disguising’ herself (often poorly) as a regular ol’ mortal pony mare (usually a pegasus, occasionally a unicorn, rarely an earth pony or thestral) to avoid most unwanted attention - her false identity of ‘Cephalanthera’ isn’t foolproof in the slightest, but it’s much better than her usual form at being discrete among crowds.
It’s not her only false identity (she has a few, most of them looking less like Glados than Ceph does), but it’s her most frequent guise as it uses the least energy to create and maintain.