To determine whether the target creature gets +3/+0, look at its creature types after you’ve gained control of it, untapped it, and given it haste. If it’s not a Goat at this time, it won’t get +3/+0, even if it becomes a Goat later in the turn. If it is a Goat, it gets +3/+0, even if it stops being a Goat later in the turn.

Goat is a creature type. You won’t see this creature type printed on creatures in this set, but keep in mind that creatures with the changeling ability are Goats.

Goatnap can target any creature, even one that’s untapped or one you already control.