There are 7 of these to get and here are their locations and in the order that you should find them, they are as follows:Faerie Familiar
This one is located in the Library section of the castle, when you are in the biggest room in the Library on the steps change into a bat and fly to the left onto more steps, in the room to the left you will find an Antivenom, a Potion, an finally the Faerie Familiar Card.Bat Familiar
Go back to the room where you previously fought Slogra & Gaibon, on the right side of the room change into a bat and fly upwards and go into the room to the right, break the glass orb and collect the Bat Familiar Card.Sword Familiar
Head to Olrox's Quarter's, in the large room containing a fountain, in the bottom left of the room head left into a long hallway which leads to the Royal Chapel, near the end of this hallway there is a breakable ceiling, you need to equip a weapon which can swing upwards and forwards such as a mace type weapon, or a sword that "slashes", the axe sub-weapon works perfect for this, a weapon such as a rapier or stabbing weapon will not work for this. Break the ceiling and go through it, head right to find the Sword Familiar.Demon Familiar
Make your way to the Abandoned Mine, there's a switch off to the right in the wall that you cannot press, go down, and left past those pesky Gremlins to find the Demon Card. Hint: Go back to the switch and turn the Demon Familiar on, the Demon will now press that switch.Ghost Familiar
Make haste for the very top of the castle where you fought Dracula as Richter in the beginning of the game, at the beginning of the broken stairs, change into a bat, fly upwards, go into the room on the right where you will find 2 life maxs', 2 heart maxs' and the Ghost Familiar Card.
Now the original Japanese version contains an additional 2 familiars that were cut out in the North American version of the game, they are as follows:Fairy Familiar
: This one is a bit tough to explain, go to Olrox's Quarter's and is in another breakable ceiling I suggest opening this map to help you http://shrines.rpgclassics.com/psx/castlevsn/map.shtml
(It will say sword card on the map)Nose Demon Familiar
Go to the Colosseum, in the uppermost hallway containing the skeleton blademasters, in the beginning of the room above there is a breakable ceiling, and again, you will need a weapon such as a mace type weapon or the axe sub-weapon to break it open, go into the room above to get the Nose Demon Familiar. Voila! Achievement unlocked.
For those of you who prefer a visual guide, here is a video showing you the 5 familiars, however they do not contain the additional Japanese familiars. (I couldn't find a video pertaining to the Japanese familiars, sorry). Credits go to Smeer516 for the video.