From its appearance on, Loveless refers to his smaller weapon as a "tank." While it may qualify as what we would call a tank, that term wasn't used until World War I and comes from hiding them as "water tanks" to conceal what they really were from the enemy during building and shipping.
Well, it's possible Loveless coined the term himself. Obviously we didn't have robot tarantulas in the "real" history, so a little variation is possible tech- and terminology-wise. If he invented a tech in the 1880's, why couldn't he call it a "tank" in the 1880's? But I believe _other_ characters call it a "tank" as well. How would they know what Loveless calls it, or just happen to come up with the same name?