I am reminded of a spot on graphic a friend (who is pregnant with her first child... a girl) posted on facebook recently. It was a decision tree diagram for girl vs boy toys.
Do you operate the toy with your genitals?
Yes ---> Stop. This is not a toy for children.
No ---> This toy is appropriate for either boys or girls.
Seriously. You asked him what he wanted and he told you. If you aren't going to get him that, why did you ask? Ponies are animals. Girls don't have a monopoly on liking animals. Period.
That being said (and the answer I think you're looking for) YES... this is totally normal and common. At this age, kids don't have the gender role hang-ups adults do and it's normal for them to like what they like without thinking about which gender it's meant for... also, totally normal for kids to like what their siblings like regardless of gender. DS (7) LOVED everything about merpeople for like a year and a half... EVERYTHING... he wanted to wrap his legs in the swimming pool etc., now he's obsessed (in exactly the same way) with Power Rangers. Who's to say which one is for boys?
Relax and get him the toy he wants.