Yes they probably can see them.

Due to their classification as XXXX, only experienced wizards (or Hagrid) should try to handle Thestrals. Breeding as well as owning these beasts may be discouraged or even illegal without Ministry consent; in fact, wizards that live in areas not protected against Muggles are forced by law to perform Disillusionment Charms on their Thestrals regularly.

From FBAWTFT pg 42

.."and the rare Thestral(black, possesed of the power of invisiblity, and considered unlucky by many wizards). As with the Hippogriff, the owner of a winged horse is required to perform a Disillusionment Charm upon it at regular intervals....

Also, read the M.OM. Classifications on pg.xxii A winged horse has a classification of xx-xxxx

xx Harmless/may be domesticated

xxx Competent wizard should cope

xxxx Dangerous/ requires specialist knowledge/ skilled wizard may handle

This is just to understand the classes. A winged horse is a Thestral and why would a charm be needed if muggles can't see them.

This is related.

Yes they probably can see them.

Due to their classification as XXXX, only experienced wizards (or Hagrid) should try to handle Thestrals. Breeding as well as owning these beasts may be discouraged or even illegal without Ministry consent; in fact, wizards that live in areas not protected against Muggles are forced by law to perform Disillusionment Charms on their Thestrals regularly.

From FBAWTFT pg 42

.."and the rare Thestral(black, possesed of the power of invisiblity, and considered unlucky by many wizards). As with the Hippogriff, the owner of a winged horse is required to perform a Disillusionment Charm upon it at regular intervals....

Also, read the M.OM. Classifications on pg.xxii A winged horse has a classification of xx-xxxx

xx Harmless/may be domesticated

xxx Competent wizard should cope

xxxx Dangerous/ requires specialist knowledge/ skilled wizard may handle

This is just to understand the classes. A winged horse is a Thestral and why would a charm be needed if muggles can't see them.

This is related.

Yes they probably can see them.

Due to their classification as XXXX, only experienced wizards (or Hagrid) should try to handle Thestrals. Breeding as well as owning these beasts may be discouraged or even illegal without Ministry consent; in fact, wizards that live in areas not protected against Muggles are forced by law to perform Disillusionment Charms on their Thestrals regularly.

This is related.

From FBAWTFT pg 42

.."and the rare Thestral(black, possesed of the power of invisiblity, and considered unlucky by many wizards). As with the Hippogriff, the owner of a winged horse is required to perform a Disillusionment Charm upon it at regular intervals....

A winged horse is a Thestral and why would a charm be needed if muggles can't see them.

Yes they probably can see them.

Due to their classification as XXXX, only experienced wizards (or Hagrid) should try to handle Thestrals. Breeding as well as owning these beasts may be discouraged or even illegal without Ministry consent; in fact, wizards that live in areas not protected against Muggles are forced by law to perform Disillusionment Charms on their Thestrals regularly.

From FBAWTFT pg 42

.."and the rare Thestral(black, possesed of the power of invisiblity, and considered unlucky by many wizards). As with the Hippogriff, the owner of a winged horse is required to perform a Disillusionment Charm upon it at regular intervals....

Also, read the M.OM. Classifications on pg.xxii A winged horse has a classification of xx-xxxx

xx Harmless/may be domesticated

xxx Competent wizard should cope

xxxx Dangerous/ requires specialist knowledge/ skilled wizard may handle

This is just to understand the classes. A winged horse is a Thestral and why would a charm be needed if muggles can't see them.

This is related.