NO! they can't its physically impossible for them to as their gullets are a one way system hence why they are prone to colic as the air can't come up it can only go down! i expect the 'burps' you heard were just air that was only in the upper oesophagus and hadn't yet reached the stomach.
QR - it is physically impossible as lizzie says but my horse once made a noise so like a loutish belch that he is officially considered the 'beer swilling lad' of the yard!
My mare does this when she comes in off grass. It sounds just like a burp so as lizzieuk1 says it must be wind in the upper part.....
does sound funny though
Mine burps if he eats his dinner to fast, i think it's just when they get air stuck inbetween their food when they swallow and it is pushed back up. His tummy sometimes make's like indigestion noises too!