HI Sara,
Animal insulins are available in pen-fill cartridges, not pre-filled pens as far as I'm aware. The Owen-mumford autopen, and the Lily Humapen Luxura can be used with hypurin cartridges. The needles for these are small & fine, not sure of exact lengths though.You can contact Wockhardt who manufacture the insulin
http://www.wockhardt.co.uk/index.asp They are really helpful and will be able to give you more updated info.
As for how you'll get on with animal insulin and how it will affect your control; you will only find out by trying it. Personally it's the best I've ever been on, and given me stable control, which i hadn't had since diagnosis 21 yrs ago. Even if my control wasn't so good, it'd still be worth it to feel human and not like the walking dead!
If your tiredness is related to Lantus/synthetic analogues, then there's no reason you won't suffer other side-effects as many of us have- these can take from months to years to show themselves and include joint/muscle pain, cognitive dysfunction, and symptoms almost identical to that of ME,Fibromyalgia & Chronic Fatigue Syndrome.
Not all people suffer these probs, but for the many that do it can have a devastating impact on their lives.
You could always give animal insulin a try for a month or 2, and see if you feel any better. If not, you can change back to Lantus.
Best of luck with whatever u decide to do, keep us posted
Jus