help!!

so my dog sparky and two cats zeus and luna get along pretty well they don’t fight or bother eachother. well recently my dog has been trying to attack my cat luna but he’s real friendly with the other one. luna will be laying down relaxing and once he sees her he gets aggressive. she’s real scared of him and won’t fight back but just run away since she’s still little. he is old! he just turned 17 in december, i don’t know what’s gotten into him recently and I want them to be friendly with either once again. but i’m not sure what to do. if he sees luna getting next to me or my sister or my other cat zeus he gets real protective and he’s never been like that.