Link Digivice is the prototype Digivice before it was completed in the next episode. According to Samuel, one of the Kurata's Dark Digivice was stolen then redesigned as Link Digivice (or L.I.N.K., Light-Incapable-Normal-Ki). He used his ki to change Ridermon into Falcon Form. It also can be useful to new power-ups called Weapon Change Mode instead of digivolution.
After Boukenger/Overdrive and Kivata, Samuel took Kurata's soldier's Dark Digivice and redesign as Link Digivice.
The one currently Digimon is Ridermon who already been accidentally created.