I don't know what it is either, but it's not arborescens, which would have much longer leaves. There are about 500 different aloe species (not to mention hybrids) most of which I've never seen. In habit it's a little similar to Aloe distans, which may be treated as a subspecies of mitriformis depending on the source. But not a good match. The growth habit is similar. There is another one called arenicola (also not a match) with the same basic pattern. These are shrubby, creeping aloes which usually grow sideways once they are old enough to have some stem. So there is going to be an ongoing support issue if you want to keep them vertical.
On the subject of vertical, the plant has recently been growing in a lower light situation than it was before, which I can tell by looking at the distance between successive leaves along the stem (internode). The top leaves are much further apart than the leaves lower down, and actually they and the stems at their base are a different color too. That's due to low light. For the longest practical lifespan indoors in a pot (before sprawling sideways behavior takes over) provide the strongest light possible, like right by your sunniest south-facing window. Even then this plant will periodically require restarting from cuttings, which should not be particularly difficult.
