My cats are very used to seeing ghosts in my apartment, so used to it that they just state with interest and don't get scared any more. I think they are either seeing angels (angels will gather wherever you display their images, something I read once and found out is very true) or some friends (like Ora Ray Glore, the fmr. Hells Angels LACO Chapter president who's the older brother of my former fiance and adopted me as his "Lil' Sis" and sometimes accompanied by his parents one of whom knew me when I was affianced), and relatives (the Two Grandmas Nellie and Kay and the Grandpas Ron C. & Jacob Russell D.) who like to hang around and watch me and the kitties play.
If your animals are frightened of what they are seeing, it's time to consult a psychic or medium who can help you deal with the spirit that is frightening them; it might be someone who is malevolent or just doesn't know they are dead and needs a little help moving on!
I rarely sense something evil in my apartment; if I do, I will read the 15th Psalm aloud and with reverence, which anything negative simply cannot remain in the presence of even one word of the Psalms spoken out loud, especially those Psalms (the 91st Psalm, Michael's psalm, also works like that, too) I also burn frankincense, yes, the real stuff they burn in church, it can be bought online if you do a google search...be sure to buy the charcoal "cups" to burn it on as it (the incense) comes in a bag of chunks and not in stick form.
Hope this helps!