A lot of that artwork is from private commissions HPG did back in the day. Whenever we find such a card, we add it to the database. The only way to find out if an artwork is ACTUALLY unused is to ask HPG.
...Which I did for Baka Kawaii Kissu, because I've been thinking about that one myself! 😜 Based on the art style, I think it was actually meant to be part of the Cherry Blossoms Under Hoof fic from Core Deck. I would put that art on a Ship card and give it a title more reflective of the scene, like "Romantic Rivals" or just "Rivals". The way I see it, Dash and Trixie are both trying to capture the heart of the same pony (the one Dash is talking to on Tsundere Rainbow Dash), and in this scene, Trixie has been waiting in Dash's path to tell her she has no chance because Trixie is prettier/refined/richer than she is (throw in a Fufufu or an Ohoho for good measure). I haven't watched anime in a long time, so I'm open to suggestions with the flavor text/title. This card's been on the back burner for a while, but since you also have an interest, let's revive it.
Another one I'm working on is the unused Fluffle Puff art (also going on a Ship card), which I asked Fluffy Mixer about at BronyCon, which reminds me I need to get in touch with him to see if he has an idea for flavor text.
Oh, and Cutie Mark Criminals was made by Preston Moore (@TailsFox88 on Twitter).