In order to make it more viable in link format I have two suggestions
(1) Allow the monster equipped to be special summoned
(2) Make it once per turn, so you can equip multiple monsters over a number of turns
I am a bit confused about the suggestions, but I will attempt to understand by making a card effect supposedly similar to what you are suggesting to see if we are on the same page...
** [Equip Spell]
Activate this card by Special Summoning 1 "Harpie" Winged Beast monster from your hand, and 1 "Harpie's Pet" monster from your hand, Deck, or GY, and then equip this card to "Harpie" Winged Beast monster. While you control a Dragon monster, your opponent cannot target the equipped monster by card effects. You can only activate 1 "Harpie's Dragon Tether" per turn.
So that is the first suggestion... But, I am kind of lost how I would approach the 2nd suggestion in tandem with this one. Do you mean like give it a once per turn effect where you Summon 1 "Harpie's Pet" monster separate from the Summoning of the Harpie Lady?