The panther is summoned by one of Drizzt's Utility Powers. Snort (the warthog) is summoned by one of Athrogate's. The ones with the same name as your party members aren't mentioned anywhere in official materials AFAIK; they're just there for homemade variants.
I suppose you can use the other ally cards if you have too few players?
Not by the rules. You seem to assume that the game is harder with fewer players, but remember that villains activate on every player turn, and with more heroes, you also have more monsters. I think the game is fairly balanced with any number of players.
I use allies as mercs that you can hire before any adventure. you can also equip or heal them. Do the math for the merc price: HPx400 gold so you can hide an ally with 3hp for 1200gp. Or you can add the Familiar voice encounter card in the mix. I use some "side quests" made by me and one of them is rescue an ally, after that quest you can use it for the main goal of the dungeon
Do you need more card ideas for the D&D Adventure System games?
I tried to insert some allies into the room event decks for each game. NinjaDorg and GeckoTh deserve most of the credit for the ally idea. I think Katrina was part of NinjaDorg's original Ravenloft deck.