There was this show I used to watch on Jetix, in India, around the 2000s. The lead was a boy and wore a red shirt, or there was something red about him. People used to fuse with their companion animals. This boy fused with a monkey and transformed into a huge King-Kong like thing. The villain had a cockatoo and when he fused with him it was a huge legged bird with wings. The lead boy had a red transformation theme color and this villain had an off-yellow theme. Unfortunately I don't remember much now, please help me find it.
Is it maybe Kong: The Animated Series (2001)?
The way I remember it, there was 3 protagonists, and the main character could fuse his mind with his King Kong companion whenever they needed to fight huge monsters, by using some kind of device if I remember.
Similarly, he could also fuse the kong into himself whenever it was inconvenient (like when they had to travel by vehicle, since the huge kong couldn't get in).
Here's a poster below:
There aren't much similarities beside that, but I thought I'd post it just in case.