In the update it's supposed for all entities to go through portals.
Well, you can't use a portal if you are riding a pig
Steps to Reproduce:
1. Create a nether portal
2. Spawn a pig
3. Put a saddle on the pig
4. Use carrot on a stick to lead the pig to the portal
5. You will pas through the portal and even if you stop you won't feel the portal effects
Related issues
is duplicated by
Comments

Related to/duplicate of MC-67?
Confirmed on 1.4.2-PRE