Can you tell whether that shoot originates from below or above the graft point? If it originates from below the graft, it should be cut off.

Can you post a photo?

Wish I could take a leaf comparison, but it's all gone.

Another stuff to note is that this branch is growing straight up beside running parallel to the ground like the other branch.

If you guys can't tell yet, I'm completely new to this and I'm not even sure it's an Acer. However, the leaves did look similar.

There is no conspicuous graft union in your photo to tell us the basal sprout has emerged below a high graft. It's normal for basal sprouts to be more vigorous than higher branches, and for them to act as a stronger energy sink. IOW, the plant receives chemical messengers from the strong basal sprout that cause it to channel more water/nutrients to the sprout than to other branches; so it's also not unusual that the leaves would be plumper/greener/larger ..... If the leaves are shaped differently or show themselves to be a different color, you can be pretty sure you're dealing with two trees - the understock & the scion.

Based on what we can see of the picture, I'd say the two near horizontal branches and the basal sprout are of the same genetics and the difference in leaf appearance is probably due to the superior vigor of the sprout. The difference in the bark appearance is just age related.

A bonsai artist would sever the main trunk flush with the top of the basal sprout, as well as the right fork of the slingshot formed by the basal sprout. Landscapers would cut the basal sprout off flush to the branch collar (not to the trunk) and make sure they regularly rub off any adventitious shoots occurring at/near the would site. It's difficult to get a good feeling for what the rest of the tree looks like, but it might also be appropriate to remove the other horizontal branches in the picture - at least eventually. I'd reserve the right to change my mind on that though, after I saw the tree in its entirety.

Al