I think its not a bad starter. I started with Neptune which is EXTREMELY useful @ all bosses. The only drawback of Neptune I could ever think of would be you can no longer poison the boss after its poisoned critically. Horus is not a bad starter, the X4 damage bonus is hard to trigger if you are not playing the multiple types. That would be a little bit hard to set up in my opinion.
Horus is probably one of the best starter REM pulls you could get. It takes a little bit to get used to matching but once you do you'll be able to set off his ability with ease. With a friend using another Horus, you're looking at 16x damage consistently. Definitely hold onto him.