I have had Poco for over 6 years now. He is almost 12 years old. I just got a new mare at the beginning of February, Cheyenne. She is 2 1/2 years old. Poco absolutely hates Cheyenne. Any time she walks by he rears up, pins his ears flat against his next and just tries to grab her with all his might. Poco has never, and I mean NEVER, been aggressive towards another horse whatsoever. But boy does he not like little cheyenne being with me! As long as I am with him (leading him, or riding him) hes okay. But as soon as I take my concentration and direct it to her its all I can do to back him off. They are not pastured together and they rarely come into contact with each other because of this. He has never acted this way and all I can think of is pure jealousy. He still gets worked with and rode just as much, maybe even more but he is a one man horse. Or one woman horse. I am the only one that works with him, the only one that rides him and he seems to act like he owns me. Which I dont like!!!!