This question is especially about a steed summoned by Paladin’s Find Steed or Find Greater Steed spells, with the assumption that a DM shouldn’t make their normal attacks magical. So it should apply to monstrosities, not just horses or beasts. An answer which applies to any steed/mount/pet/companion is of course better.
How can a PC’s mount’s or steed’s attacks be made magical?
Help of others in the party (so abilities of any class, as long as they can be applied to the mount of this player), as well as help of NPCs or magic items is ok.